# Another CO2 Chart to try



## hoppycalif

There have been several discussions about the inaccuracy of CO2 measurement using KH and PH. It was suggested that we take a tank sample and let it sit overnight to reach equilibrium with the CO2 in the atmosphere. That was assumed to be about 3 ppm. If the PH increased by about 1.0 the CO2 was assumed to be about 30 ppm. This led me to look at the CO2/PH/KH equation some more and reduce it to a difference between equilibrium PH and tank PH, to give the attached chart. Any comments? If this is mathematically correct, it looks like a more accurate way to measure CO2, without worrying about what the KH is.










Edit: My experimentation indicates that the equilibrium ppm for my kitchen is about 4 ppm. This chart has been revised to reflect that.


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## BryceM

I absolutely agree that the method you describe is a better way to look at CO2 concentration since it removes many of the variables that can upset the traditional KH/pH/CO2 chart. Organic acids and non-carbonate buffers are always present to some degree in a real aquarium which invalidate the equation the chart was derived from.

Your chart above should be correct if your assumption of 3 ppm of dissolved CO2 at equilibrium is, in fact, true.

CO2 (ppm) = equilibrium CO2 (ppm) X 10 ^ change in pH

I've been looking all over the web looking for reference values for dissolved aqueous CO2 levels at atmospheric equilibrium and so far I haven't found anything outside of aquarium related sites to verify this number.

For anyone that hasn't been following this discussion over the past two or three months, the basic idea is to set some aquarium water aside in a container with a large surface area, or to bubble air through the sample. If you don't aerate the sample, allow 24-48 hours for the pH to stabilize as it comes to equilibrium. The differenece in pH between the degassed sample and the water in your tank comes from the difference in CO2 levels.


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## banderbe

guaiac_boy said:


> I absolutely agree that the method you describe is a better way to look at CO2 concentration since it removes many of the variables that can upset the traditional KH/pH/CO2 chart. Organic acids and non-carbonate buffers are always present to some degree in a real aquarium which invalidate the equation the chart was derived from.
> 
> Your chart above should be correct if your assumption of 3 ppm of dissolved CO2 at equilibrium is, in fact, true.
> 
> CO2 (ppm) = equilibrium CO2 (ppm) X 10 ^ change in pH
> 
> I've been looking all over the web looking for reference values for dissolved aqueous CO2 levels at atmospheric equilibrium and so far I haven't found anything outside of aquarium related sites to verify this number.
> 
> For anyone that hasn't been following this discussion over the past two or three months, the basic idea is to set some aquarium water aside in a container with a large surface area, or to bubble air through the sample. If you don't aerate the sample, allow 24-48 hours for the pH to stabilize as it comes to equilibrium. The differenece in pH between the degassed sample and the water in your tank comes from the difference in CO2 levels.


Well now you're assuming we all have access to accurate pH readings. I use an AP pH test kit and a Red Sea pH test kit. Sometimes they agree, sometimes they are off from the other by 0.2.


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## acbaldwin

What about other things that affect your pH reading? Wouldn't they lead to a false CO2 reading?


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## BryceM

banderbe said:


> Well now you're assuming we all have access to accurate pH readings. I use an AP pH test kit and a Red Sea pH test kit. Sometimes they agree, sometimes they are off from the other by 0.2.


Yeah, even pH probes can be off by quite a bit - especially if you don't callibrate often or replace them when they wear out. Color comparison test kits can be even harder to get accurate readings out of, not to mention innacuracies in measuring KH.

Like everything else though, two decimal precision is great for labs but a little ridiculous for a hobby. I'm assuming everyone does the best with what they've got. I think even the cheapest pH test kit can distinguish between a 0.4 and 1.0 pH difference. This will at least get you in the ballpark, especially with several readings over time.

Another method that might be the best of all is to slowly crank up the CO2 until the fish look like they're not liking it. Then back off about 0.2 or 0.3 pH units. Go slow and only do it when you're around to watch the fish. As long as the plants are growing and algae isn't, there isn't much point in seeing how far you can push it.


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## BryceM

acbaldwin said:


> What about other things that affect your pH reading? Wouldn't they lead to a false CO2 reading?


Not at all..... That is the beauty of this method. If you take the water right from the tank and let it degas, the only difference between the two samples will be the concentration of CO2. The pH of both will certainly be affected by organic acids, tannins, phosphate buffers, etc, but equally so.


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## hoppycalif

This method seems to me to have only one unique disadvantage, and that, of course, is the time it takes to get a reading. But, it does appear to cancel out all of the various other errors we run into by measuring KH and PH.

Today an "old timer" who is well known in aquatic plant circles was upset that we would even consider using fish as a test device - torturing them with ever higher CO2 levels until their distress is obvious to us. That did impress me and started me to thinking harder about how else to measure CO2.


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## jude_uc

Good job! That's a very good idea to solve the problem.

-Adam


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## MatPat

hoppycalif said:


> There have been several discussions about the inaccuracy of CO2 measurement using KH and PH. It was suggested that we take a tank sample and let it sit overnight to reach equilibrium with the CO2 in the atmosphere. That was assumed to be about 3 ppm. If the PH increased by about 1.0 the CO2 was assumed to be about 30 ppm. This led me to look at the CO2/PH/KH equation some more and reduce it to a difference between equilibrium PH and tank PH, to give the attached chart. Any comments? If this is mathematically correct, it looks like a more accurate way to measure CO2, without worrying about what the KH is.


Here are my test results from a few months ago:

Tap pH - 8.51. 
Rested Tap - 7.59. 
Tank - 6.16
Tank Aerated - 7.67

That gives me about 95ppm of CO2 according to the chart. I'm thinking this chart may be fairly close. If I add fish without using drip acclimation for several hours, the fish pretty much sink to the bottom of my tank in a matter of minutes.

I will try and run some more tests on my tank water tomorrow if time permits. It would be nice if we could find a way to eliminate one more test kit that new folks will have to purchase!

I can imagine someone being upset by harming the fish, but if caught before they are unconscious, it should be temporary. Once you see them start to get stressed, you can easily back off the CO2 before any damage is done. Even the fish that sank to the bottom of my tank were revived by aerating them in a small volume of water.


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## turbomkt

One thing to note about pH testers (digital)...

regardless of calibration (accuracy), as long as the precision is fine this will work. Readings can be off by whatever amount, but they'll both be off by the same amount. An inexpensive hand held digital meter should be quite good enough and cheaper than test kits in the long run.


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## RTR

hoppycalif - that particular old timer is also the only person I know of who worked out the equilibrium levels for CO2 from air, with compensation for altitude/barometric pressure. Check out his web site, it has to be there somewhere, or on The Krib, or both.


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## yildirim

Hello friends,

Aren't you still bored of this CO2 conversations. Yes it is a must for all planted tanks but measuring the level is very tricky as there are many factors affecting all the variables. So you can not be sure that the level you read from any of the tables is 100% accurate. What I do is always try to keep it as high as possible and the only indicator for me is fish distress.

YILDIRIM


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## MarcosB

I think that 3 ppm is far from the true value of the equilibrium.
A simple calculation using Henry´s law gives a value of 0,5 ppm of dissolved CO2 in equlibrium with atmosphere.


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## John S

i agree with YILDIRIM i run my co2 as high as i can get away with as long as you are above 30 ppm u are going to be fine let the fish tell u how high it is lol. everyone gets to hung up on spot on testing.


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## MatPat

John S said:


> i agree with YILDIRIM i run my co2 as high as i can get away with as long as you are above 30 ppm u are going to be fine let the fish tell u how high it is lol. everyone gets to hung up on spot on testing.


I agree with both of you on the testing. Even with good kits and meters there are some variables, not to mention human error, and no one test will be perfect. I rarely test unless something appears to be wrong but this can be a difficult concept to grasp for those without much experience. When you are new to the hobby, it is nice to have some "numbers" to go by until you gain a bit of experience.

According to the KH/pH chart, my levels are over 200ppm (LaMotte Alkalinity test and Hanna 98129 meter) and if I were to follow the advice at keeping my levels in the 30-40ppm range, I would have massive BBA in my tank. Go ahead and ask how I know this 

My fish do not gasp at the 200ppm range (90ppm with Hoppy's method) but new fish succumb very quickly to these levels. I have been increasing the CO2 very slowly for a long time now and it appears that the fish may be able to adapt somewhat.

Raising the CO2 levels can be very dificult for someone new to the hobby to understand. I myself was a victim of this when I first started following the advice of folks to increase my CO2 levels. Go ahead and ask how I know this too  I think what Hoppy and the rest of us are trying to arrive at is a simpler way for beginners to determine CO2 levels and not kill off their fish!

If a new method can help only one person get into the hobby and stay here, then I think it is a worth while discussion


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## BryceM

Kudos MatPat,

Exactly what I wish I could have said. The same logic applies to everything that can sometimes derail those with less experience. I've kept fish long enough to know when they don't look 'right' and I can usually get out of trouble pretty quickly, but all the chemistry behind nutrient levels and CO2 had me baffled for the longest time. Net result = tons of algae & confused about what to do with it.

After getting Lamotte kits and doing some testing for a few weeks I now know exactly what high and low nitrates look like in my tank. It's much easier now to figure out what to do. I also can tell roughly what the iron situation is like just by looking at the leaves after several months of observation.

It's easy to forget that you didn't always know something. It's the same thing with CO2, but the stakes are higher, especially when you don't have experience to fall back on. Make a little mistake = algae attack. Make a big mistake = dead fish.


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## hoppycalif

The problem I ran into when I started my pressurized CO2 was that when I had 30 ppm +/- a bit, I felt I was at the max, but I had BBA starting up with a fury. So, I decided it was worth it to me to sacrifice my half dozen fish by going higher to try to stop the BBA. I did so, and the fish were fine. I kept doing so and the fish were still fine. Eventually, by removing most of the BBA and keeping a really high level of CO2, 170ppm per the chart, I stopped the BBA. But, I always worried about what I was doing to the poor fish. Then I saw guaiac boy's idea that the difference between the water in equilibrium with the air and the water in the tank should be about 1.0 ph, and that is about what I had. So, I just gave up on measuring either PH or KH. But, I still wondered. And, I got worried about telling people to jack up the CO2 until the fish objected. Only yesterday did it occur to me that I could easily reduce the CO2 chart to eliminate the KH measurement.

So, I see this as the better way to "measure" CO2 - not perfect, of course, but better.


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## Bert H

I can't see the chart, just broken link symbol.


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## hoppycalif

The image hosting site seems to be down now. I guess I will have to pick another one?


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## jrIL

Chalk me up as another who the charts did not work for. I have been very happy since using the rested tank water method as first suggested to me by guaiac_boy a couple of months ago. I continue to use my ph meter as a guidline to work from.

JR


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## MatPat

Bert H said:


> I can't see the chart, just broken link symbol.


The link seems to be working now. Maybe we can persuade Hoppy to add it to this site.


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## BryceM

One word of caution......... It's still important to keep an eye on the KH, especially if you need to supplement NaHCO3 due to very soft tapwater. My tap KH is <1. If you let it get too low, you risk a pH crash. Also, both your degassed CO2 levels and your tank CO2 levels will be quite low. Fish can compensate for elevated CO2 levels using certain metabolic pathways, and they can compensate for gradual changes in osmotic load, but at some point they just can't compensate for living in acid. Once pH starts getting down into the high 5's I think you'll see problems in most fish and some of the plants we keep.

IMO a KH of at least 3, seems prudent. I keep mine between 4 and 5.

Also, many people are reporting recently that they maintain enormously high CO2 levels to battle BBA. Their fish seem to do just fine, but when they add new ones they go into shock. It's probably something like opening the window of an airliner at 28,000 feet - people pass out pretty quick without O2, yet climbers have done Everest (29,035 feet) without O2. The trick is a gradual ascent over several weeks to allow the body time to compensate. Introducing new fish might become difficult, making a gradual transition mandatory.


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## Navi

It seems to me this theory is totally based on an assumed equilibrium of 3 mg/l. That's a big assumption IMO. If the assumed equilibrium was off by 1 mg/l either way, the chart would be totally invalid.

Or am I missing something here?


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## MarcosB

Under normal circumstances the CO2 equilibrated with the atmosphere should be 0.5 ppm, and the equation to determine its concentration as a function of pH variation before and after aeration is: [CO2] = 10^(pHdiff - 0,3).


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## banderbe

MatPat said:


> I agree with both of you on the testing. Even with good kits and meters there are some variables, not to mention human error, and no one test will be perfect. I rarely test unless something appears to be wrong but this can be a difficult concept to grasp for those without much experience. When you are new to the hobby, it is nice to have some "numbers" to go by until you gain a bit of experience.
> 
> According to the KH/pH chart, my levels are over 200ppm (LaMotte Alkalinity test and Hanna 98129 meter) and if I were to follow the advice at keeping my levels in the 30-40ppm range, I would have massive BBA in my tank. Go ahead and ask how I know this
> 
> My fish do not gasp at the 200ppm range (90ppm with Hoppy's method) but new fish succumb very quickly to these levels. I have been increasing the CO2 very slowly for a long time now and it appears that the fish may be able to adapt somewhat.
> 
> Raising the CO2 levels can be very dificult for someone new to the hobby to understand. I myself was a victim of this when I first started following the advice of folks to increase my CO2 levels. Go ahead and ask how I know this too  I think what Hoppy and the rest of us are trying to arrive at is a simpler way for beginners to determine CO2 levels and not kill off their fish!
> 
> If a new method can help only one person get into the hobby and stay here, then I think it is a worth while discussion


Ditto, I have 86 ppm CO2 according to the charts, and I know for a fact that my KH is 18 (city water report and confirmed by test kits) and I am reasonably sure my pH is at or around 6.8 (AP and RedSea kits agree).

But, three of my fish died last week. Oddly the others were fine and never showed any signs of stress.


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## R0bert

> Under normal circumstances the CO2 equilibrated with the atmosphere should be 0.5 ppm, and the equation to determine its concentration as a function of pH variation before and after aeration is: [CO2] = 10^(pHdiff - 0,3).


MarcosB can you show me how you arrived at 0.5ppm? I worked it out even lower than that, but I'm not certain about my constants.


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## hoppycalif

We should be able to measure the equilibrium concentration of CO2 when a water sample sits open all night, by using distilled or demineralized water, with a bit of baking soda in it for KH. Doing this for a couple or so different KH's should settle the question on whether this method is at all accurate. I plan to try this tonight, but I hope others can try it too. A gallon of distilled or demineralized water is pretty cheap.


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## MarcosB

Robert
The composition of CO2 in atmosphere is ca. 380 ppm (0.038 %) which gives a partial pressure of 0.00038 atm. The Henry´s law constant for CO2 is 3.38x10^-2.
Thus, using the Henry´s equation we have:[CO2]aq = 0.00038 x 0.0338 = 1.3 x 10^-5 mol / L of CO2.
Converting to mass: 
1.3x10^-5 x 44 = 0.00056 g/L= 0.56 mg/L (ppm)


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## R0bert

Thanks MarcosB. I made a mistake with the mass conversion.

The last post in this article explains the method quite clearly: http://www.thekrib.com/Plants/CO2/kh-ph-co2-chart.html


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## MiamiAG

Hobby,

Just upload the chart to our Photo Album and link to it from there. Our image hosting is free to members.

Excellent discussion by the way.


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## BryceM

I agree that the math according to Henry's law gives a value of 0.5 ppm for distilled water. Why then have people always cited 2-3 ppm as the equilibrium value for planted tanks? Does the presense of a carbonate buffer affect how much is able to dissolve? There is a disconnect somewhere - maybe the 2-3 ppm thing is just a myth, but it's been going around for a very long time.


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## dkfennell

Here is article by Carroll, Slupsky and Mahter comparing empirical results to Henry's Law's prediction on Solubility of CO2 in Water at Low Pressure:

http://www.nist.gov/srd/PDF files/jpcrd427.pdf


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## hoppycalif

Last night I went to my local grocery store and bought a gallon of water sold for steam irons, etc., labeled as RO water. I poured out about 80% of it into a large shallow bowl, added about 1/32 tsp baking soda, and waited for things to stabilize. I measured KH=2, PH = 7.2 to 7.6, getting different results (or different interpretations of the color) for three measurements taken from 2 to 12 hours later. The KH test I did with double the amount of water, so it is pretty accurate, but the PH is very iffy. Today I have mixed a small batch at about KH=1, and added another 1/32 tsp to the bowl to get KH = about 4. I will check PH/KH in about 4 hours. I'm already almost sure the actual equilibrium CO2 is more than .5 ppm and closer to 3 or 4 ppm. Remember, we don't care what the theoretical value is, just what the water reaches after sitting open to the air for 12 hours.


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## Bert H

Very interesting discussion here folks! From apd: http://fins.actwin.com/aquatic-plants/month.200603/msg00103.html where George Booth states that the theoretical equilibrium CO2 concentration is: 0.485ppm.

Interesting set of threads going on in apd as well regarding this topic.

The other thing that can play a role in this is with the kh measurement. It is my understanding that kh measures alkalinity values. If you have alkalinity not derived from carbonates, then your kh values would be artificially higher, thereby giving you artificially higher perceived CO2 values. This would explain why a lot of us (myself included) appear to have CO2 values in the 60's with no apparent fish/shrimp stress in their tanks.

Any chemists care to chime in?


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## dkfennell

One thing is certain from this discussion. You can't take at face value anything anyone says about CO2 concentration in their tank without finding out how they derived it. This applies not only to fish stress, but also amount of CO2 for plant growth. It would be a big mistake for someone estimating his own CO2 by a method that understates the CO2 concentration following the suggestions of another who overestimates it. There would be a problem (but lesser in severity) the other way too.

I also think guaiac_boy deserves alot of credit for crystalizing this issue. At least he did for me.

Darrell Fennell


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## MarcosB

Hoppycalif
There is no reason to a water sample in equilibrium with atmosphere has CO2 levels as far as 3 ppm. The equilibrium is related to CO2 levels in atmosphere, which is estimated at 380-390 ppm.
There are some possible situations where the CO2 can be far from the theoretical equilibrium of 0.5 ppm. This includes an ambient with poor external air ventilation, water with high biological activity (with microorganisms).
I do not understand how can you determine the CO2 equilibrium level with this experiment that you are developing.

Bert H
Compounds like phosphates and humic acids (and others) can interfere in the carbonate alkalinity determination. The KH value became higher than the real leading to errors when compared to pH x KH x CO2 charts.
But these compounds can also capable of influence this method based on pH. The pH variation due to the CO2 loss in a water sample that the KH is constituted only by bicarbonate/carbonate is higher than a water sample with the same KH value but with the buffering constituted of only phosphate salts.


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## BryceM

Hoppy, look at the hornets nest you stirred up.

I think dkfennel has a good point. One person's CO2 reading is not necissarily the same as another person's. I've been running ideas by a friend of mine who graduated as a chemistry major from Dartmouth. He is, without a doubt, one of the 10 smartest people I know. His conclusion is that the relationship between CO2, pH, and acid buffers in aquarium systems are too complicated to model with simple methods. There are just too many variables.

I think that a couple of ideas have merit. One is to add CO2 unitl the fish show distress and back off a bit. This method has drawbacks. I'm pretty convinced you can slowly acclimate fish to conditions that they wouldn't normally do well at. Therefore, the speed at which you drop the pH is important. The other problem is that it's not always easy to get consistently accurate pH readings, even with an electronic meter.

The other idea is to degas a water sample and plan on dropping the pH about 1.0 units. I'm not sure that I know 'exactly' what ppm of CO2 this gives me, but it seems to work and is probably a good place for a newbie to start. I do think it removes certain innacuracies inherent to the traditional KH/CO2/pH chart.

People's estimation of their CO2 is probably about as accurate as their estimation of light reaching their plants. It doesn't take much checking with light meter to figure out that watts/gallon is an absurd unit of light. It's just the most convenient one we have.


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## hoppycalif

*Revised CO2 Chart*

I don't think any of us would say that we are capable of measuring the CO2 in our water accurately enough for any scientific purpose. What we do need is a way to estimate that number accurate enough so we can compare CO2 levels among ourselves, so we can have a good starting point when we start injecting CO2 into our tank, and so various waters we use will all give the same result within a reasonable error band for the same amount of CO2. I think that is all we have ever wanted. And, that is what I found I didn't have, with my "170 ppm" reading using the Chuck Gadd chart or the many equivalent charts. So, I hope we can all view this effort as just another, possibly more accurate way to find out how much CO2 we have in our water. I finished my experiments trying to determine the CO2 ppm in water that sits on my kitchen counter for a few hours exposed to the air.

Method:
Use "purified water", from grocery store, which is RO water filtered through a carbon filter. Add only sodium bicarbonate to adjust carbonate hardness. Pour water into a large shallow glass bowl and add baking soda, stir for a few seconds and wait for 2 to 12 hours to test KH and PH using AP test kits. Room temperature was about 72 degrees F for all tests.

Initially water was KH=2, from adding about 1/32 tsp baking soda to about .8 gallon of water. Then a small glass of water was removed and diluted about 50-50 with more water from the grocery store bottle, to get KH=1.5. Later that was diluted with about a third more grocery store water to get KH =1.0. The water in the bowl then had another 1/32 tsp of baking soda added to get KH=3.5. So, the range of KH tested was 1.0, 1.5, 2.0, 3.5.

Problems:
Getting an accurate reading of PH was nearly impossible, since all of the PH's were in the range of 6.9 to 7.6 where the color differences are very hard to be sure about. KH was fairly accurate, since using 10 ml of water instead of 5 ml, doubles the accuracy from about +/- .5 to +/-.25.

Results:
KH=1.0 CO2=3.9 ppm
KH=1.5 CO2=4.5, 4.5 ppm
KH=2.0 CO2=2.4, 3.8 ppm
KH=3.5 CO2=4.2, 4.2 ppm

Assuming all results are equally trustworthy, the average is 3.9 ppm.
Omit the 2.4 reading, and the average is 4.2 ppm.
So, I am going to assume that the equilibrium CO2 concentration for water exposed to air at about 72F for 2 to 12 hours is 4 ppm.

This changes the chart I posted before, but given the accuracy most of us have in measuring PH, the difference isn't great. Here is the new chart, which I will use until someone finds and posts a better way to do this:


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## RTR

hoppy - don't forget altitude above sea level. Barometric pressure _does_ matter. George Booth is way up in the Rockies and makes sure to mention that fact when he is talking CO2 in his tanks versus other's tanks.


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## hoppycalif

Since the ppm of CO2 this chart gives you is directly proportional to the "equilibrium" value for ppm of CO2 when a sample sits for a few hours, he, or anyone else could just buy a gallon of RO water and repeat the test to get their own value, then multiply the numbers in the chart by their value divided by 4. And, I agree that barometric pressure would have to be a factor, but I have no idea how much it would change it, especially considering the fact that it is hard to accurately measure PH in any case.


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## Glouglou

*Hanns-J Krause CO2 Test*

*Hanns-J Krause CO2 Test*
Author Handbook Aquaria Technology
Krause recommends a different method to measure CO2 content. That method is insensitive to the presence of unusual buffers.

*Step 1:*
Take another water sample with some pH indicator and run the hose from an air pump into the sampe for a few minutes. This sets the CO2 level at 0.6ppm.
->Measure pH of the water (=X).

*Step 2:*
Take small water sample with some pH indicator. Stick a straw into the water and exhale through the straw into the water sample for two or three minutes. This sets the CO2 level at 60ppm.
->Measure pH of the water (=Y)

*Step 3:*
The optimum CO2 concentration of 10-20ppm is at the pH value about 2/3 of the difference between X and Y:
*pH,optimum = X +.67*(Y-X).*

As an example, if you measure pH 8 with the first sample, and pH 6.5 with the second sample, a tank pH of 7 corresponds to 30ppm CO2, regardless of any buffers that might otherwise confuse a CO2 test or distort the charted values.


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## dkfennell

Gouglou, if we assume de-gassed aquarium water has concentration of .6 ppm CO2 (from Henry's Law, assuming room temperature (?), and sea level (?)), then why in sample (I assume also from the aquarium) do you add additional CO2 (from breath) and make an assumption about the concentration of CO2 in that water (before testing pH)? Wouldn't simply testing the actual aquarium water, as guaiac_boy long ago suggested, and using MarcosB's equation (on page 3 of this threat) be a better approach, at least, for no other reason, one fewer assumption is made?

The only argument I can see in favor of your approach is that the wider gap in pH difference in your test makes testing the difference (which color pH tests) slightly easier (or, perhaps better, the larger difference makes the imprecission of the color tests contribute less error to resulting estimate). But I don't see how that compensates for the additional assumption.

Darrell


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## IUnknown

You could adjust the chart to compensate for any buffers in the tank 
using Tom's suggestion. I need to dial in the Co2 on my new tank, so I'm 
going to try a couple of these methodes to see how they compare. The 
one thing I need to look into (I'll report back if I find an answere) is 
how the Co2 test kits work (Like Amano's eye dropper, or the lamotte 
kits). Are they affected the same way?


> You can do a dirty little method to figure out your CO2 relatively close(pH is log scale though but this method is close enough in general)
> 
> Take a tap water sample, lt it sit 24 hours, measure pH/KH, should be about 3.7ppm CO2 or so.
> 
> Take a tank water sample, let it sit for 24 hours, take a pH/KH reading. Assume the CO2 content in both are equal.
> 
> Take the difference in the pH, say 0.4, add that from the tank reading.
> 
> So a pH of 6.4 is really 6.8 etc.
> 
> This is not perfect, but it's doable and relatively close.
> You can transform the scale to log scale to get a closer approximation but the accuracy gained from all that is still low.
> 
> But since folks do large water changes, the humics are really regulated to the substrates and much less so to the water column after a few water changes.
> 
> I highly recommend for folks like you guys to do the add enough CO2 to get the plants all happy and then a little more, I hate controllers personally.
> 
> Test kits and other things cause more problems in some ways.
> Simple is good. I'm still leary of suggesting this method though to new folks, I've seen dead fish tanks and if you kill the fish, I do not care what folks say, that's bad advice if there's a high risk of a new person doing that.
> 
> Many folks in Asia do that I see many post on that. I see that here in the USA also. But I suppose it's better than algae? hehe
> Amano's done it too. I've been lucky, I killed some shrimps once, no fish to date yet.
> 
> Regards,
> Tom Barr


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## Bert H

Aren't we all coming up with the fact that a drop in pH units of 1.0 from atmospherically equilibrated water equals approx 30ppm of CO2 in your tank? Or am I missing something here?


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## hoppycalif

Bert H said:


> Aren't we all coming up with the fact that a drop in pH units of 1.0 from atmospherically equilibrated water equals approx 30ppm of CO2 in your tank? Or am I missing something here?


In my opinion, based on my testing, a drop of 1.0 pH units equals nearer to 40 ppm of CO2. But, I doubt that any of the methods we use is more accurate than +/- 5 ppm, if that.


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## BryceM

Since it's a logarithmic scale, a pH drop of 1.0 unit will mean that you have 10x as much acidity (CO2) as you did before. The final concentration depends on the actual starting concentration, which can be a bit difficult to determine. Most people have assumed something in the 2-3 ppm range - where this number comes from is not clear at all since Henry's law predicts much less (but this is for a non-buffered, distilled sample of H2O).

When it gets right down to it, figuring out exact ppm's of CO2 is a little tricky, but from a practical point of view, dropping one's pH by 1.0 unit by adding CO2 is reproducible from one person's setup to another's and it seems to add enough to grow plants, keep algae at bay, and not kill too many fish. That's good enough for me.

One word of caution. Dropping your pH by 0.5 will result in 3.2x the initial level. Dropping it by 1.0 gives 10x initial, and dropping it by 1.5 gives you a whopping 31.6x initial - more than 3x as much as a 1.0 unit drop. Once you get up in the neighborhood of a 1.0 pH unit drop, going any lower equates to a lot more CO2 very quickly.


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## hoppycalif

The best part about the PH drop of one unit is how easy it is to remember, and it gets you almost exactly where you want to start. From there you can make small changes in bubble rate and adjust to what the fish like best and the plants respond well to. Until the folks at Seachem or Aquarium Pharm. invent a good, easy titration method that works no matter what else is in the water, this may be the best we can do.


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## Edward

How much more ‘acid’ do we need to lower pH by 1 in water at 0.5 KH versus 5 KH?


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## BryceM

Edward said:


> How much more 'acid' do we need to lower pH by 1 in water at 0.5 KH versus 5 KH?


I'm not sure I understand your question, but I'll assume you mean CO2 by 'acid'. The relationship between CO2 concentration and change in pH between tank and degassed samples is independent of the KH of the water. What this means is that a pH drop of 1.0 will indicate the same concentration of dissolved CO2, regardless of the water's buffering capacity.

Why this should be true was not at all intuitive to me. I initially supposed that the pH of water with little buffering capacity could be dropped quite rapidly by only a little CO2, while highly buffered water wouldn't be affected until one added truly large amounts of CO2. I think the chemistry behind the explanation has something to do with the fact that CO2 is in equilibrium with it's own buffer in an aqueous solution:

CO2 + H2O <--> H2CO3 (carbonic acid) <--> H+ (proton) + HCO3- (bicarbonate)


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## Edward

guaiac_boy said:


> The relationship between CO2 concentration and change in pH between tank and degassed samples is independent of the KH of the water. What this means is that a pH drop of 1.0 will indicate the same concentration of dissolved CO2, regardless of the water's buffering capacity.


How is that possible? Look at the page 4, 5 and 6 in *this article* or any other KH / pH / CO2 table.

1 KH / 6.0 pH = > 30 ppm CO2
1 KH / 7.0 pH = > 3 ppm CO2
Increase of 27 ppm CO2

10 KH / 6.0 pH = > 300 ppm CO2
10 KH / 7.0 pH = > 30 ppm CO2
Increase of 270 ppm CO2

Clearly the amount of CO2 is dependent on the KH. Am I missing something?

Thank you
Edward


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## BryceM

Edward, you're not starting from a degassed value. The first 1.0 unit of pH drop AWAY from the degassed value will give you 10X the concentration of CO2 at atmospheric equilibrium. If you look at the second 1.0 unit of pH drop you'll get 100X the concentration of a degassed state.

If you consider KH 10, pH 7.0 you're already got a ton of CO2 in solution, enough that it's already about 1.0 pH unit away from a degassed state. If you go down still another pH unit, of course you'll see a huge jump in CO2 levels.


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## RTR

The one log (pH drop of one unit) drop applies *only* from a baseline of air-equilibrium tank water. It does not apply to further additions of CO2 beyond one unit. I believe it has already been pointed out that going past a full log drop involved massive CO2 concentrations on the water.

In the second pairing of Data points used in your example, the air-equilibrium value would have to be on the order of pH 8.0 to have 30 ppm at pH pf 7.0


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## Mnemia

guaiac_boy said:


> I'm not sure I understand your question, but I'll assume you mean CO2 by 'acid'. The relationship between CO2 concentration and change in pH between tank and degassed samples is independent of the KH of the water. What this means is that a pH drop of 1.0 will indicate the same concentration of dissolved CO2, regardless of the water's buffering capacity.
> 
> Why this should be true was not at all intuitive to me. I initially supposed that the pH of water with little buffering capacity could be dropped quite rapidly by only a little CO2, while highly buffered water wouldn't be affected until one added truly large amounts of CO2. I think the chemistry behind the explanation has something to do with the fact that CO2 is in equilibrium with it's own buffer in an aqueous solution:
> 
> CO2 + H2O <--> H2CO3 (carbonic acid) <--> H+ (proton) + HCO3- (bicarbonate)


I'm not sure I fully understand the chemistry here either, so let me see if I follow what is being said.

Le Chatelier's Principle is the relevant thing here. The part of this that is important here is that when you change the concentration of some part of this equilibrium system then the system will "shift" in the opposite direction to compensate for the change. So when we add CO2 to our tanks, the system shifts to the right to produce H+ ions (driving down the pH) and HCO3- ions. When we "degas" the water sample, the system shifts back in the other direction, and the number of H+ ions decreases (increasing the pH).

From what I can tell, there are two complicating factors here: the presence of the -HCO3, which also obviously affects this system because it drives the reaction to the left (increasing the pH by reducing [H+]), and the presence of any other acids (which also will contribute H+ ions, but in their own linked equilibrium system). This seems potentially very complicated, hence the reason for trying to improve on the original KH/pH chart.

Now, when you degas the sample you are bringing the CO2 system into equilibrium with the atmosphere. The atmosphere drives CO2 into the water at a certain rate (which is known). The pH that will result does depend on the amount of bicarbonate in the water (I believe it is 5.65 IIRC if no bicarbonate is present). But, the bicarbonate should be consumed at the same rate as the H+ ion when you degas the sample, if there aren't any other complicating factors. So (if I understand correctly) the actual concentration of it shouldn't affect the change in pH, but only the absolute pH at each endpoint.

It is true that if you have a lot of -HCO3 in your water that more CO2 will be required to change the pH a given amount (this is the "buffering" we talk about - the excess -HCO3 drives the equilibrium to the left). But when we remove all the excess CO2, by degassing it, a larger excess of -HCO3 will drive the back pH up further than a smaller excess, and we get the same change. So the total change should be the same regardless, since the only thing we are changing is the CO2 concentration.

Correct?


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## BryceM

Mnemia,

It is very complicated, like most things in life . I really think this method of looking at a pH depression from a degassed state removes a lot of the variables from the equation.


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## Edward

Y = gassed tank water 6.5 pH 
X = degassed tank water 8.0 pH
__________________________

at 1 KH, 10 ppm CO2 - 0.3 ppm CO2 = *9.7* ppm CO2
at 5 KH, 49 ppm CO2 - 2 ppm CO2 = *47* ppm CO2
at 10 KH, 97 ppm CO2 - 3 ppm CO2 = *94* ppm CO2

So what is it, 9.7, 47 or 94?
How does the X + 0.67 x ( Y - X ) help here?


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## hoppycalif

I have changed the image hosting for this chart, so it now shows up on the first post. In addition I revised the chart to reflect that when I did some experimenting, the equilibrium CO2 ppm a water sample on my kitchen counter reached was about 4 ppm, not 3 ppm. This chart is only an approximation of the amount of CO2 in the water, but I don't believe we have a means available to get a more accurate measurement.


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## BryceM

Edward said:


> Y = gassed tank water 6.5 pH
> X = degassed tank water 8.0 pH
> __________________________
> 
> at 1 KH, 10 ppm CO2 - 0.3 ppm CO2 = *9.7* ppm CO2
> at 5 KH, 49 ppm CO2 - 2 ppm CO2 = *47* ppm CO2
> at 10 KH, 97 ppm CO2 - 3 ppm CO2 = *94* ppm CO2
> 
> So what is it, 9.7, 47 or 94?
> How does the X + 0.67 x ( Y - X ) help here?


I'm not sure I understand your question...

If you change the KH of the water, the water's degassed pH will change too. You can't make the assumption that the degassed pH will remain constant at differing pH values - it doesn't.

Right now my tank has a KH of 4, a degassed pH of about 7.4 and a 'gassed' pH of 6.4. This makes by pH drop about 1.0 units.

If I were to add some NaHCO3, my KH would go up, and degassed pH would also go up. My target pH would also then go up, since I'd be achieving my 1.0 unit drop at a higher pH. If I still aimed for 6.4, my CO2 concentration would be higher than before.

Or am I completely missing the point of your post?


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## Navi

I think I can see Edward's point.

A 1.0 drop in pH from 8 to 7 should yield 30 mg/l CO2 if the degassed water reaches an equilibrium of 3 mg/l. However, Hoppy's calculations would put the CO2 level at 40 mg/l, because his tests resulted in an equilibrium level of 4 mg/l. 

What if the equilibrium level was 2 ppm? This would give a CO2 level of 19.5 mg/l. So in theory, the CO2 level could be somewhere between 19.5 -40 mg/l for a 1.0 drop in pH.

I don't think there is a fool proof method of testing CO2 for the hobbyist.


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## Mnemia

Navi said:


> I think I can see Edward's point.
> 
> A 1.0 drop in pH from 8 to 7 should yield 30 mg/l CO2 if the degassed water reaches an equilibrium of 3 mg/l. However, Hoppy's calculations would put the CO2 level at 40 mg/l, because his tests resulted in an equilibrium level of 4 mg/l.
> 
> What if the equilibrium level was 2 ppm? This would give a CO2 level of 19.5 mg/l. So in theory, the CO2 level could be somewhere between 19.5 -40 mg/l for a 1.0 drop in pH.
> 
> I don't think there is a fool proof method of testing CO2 for the hobbyist.


That's true, but isn't the CO2 equilibrium constant with the atmosphere a known quantity? I did some searching on Google and there are chemistry websites giving an absolute figure (seemingly the same everywhere) for CO2 concentration at equilibrium with the atmosphere. These numbers are calculated from the concentration of CO2 in the atmosphere and an equilibrium constant with water. This should be a known and scientifically derived number and not something we have to guess at. The only thing is, it varies as a result of things like barometric pressure and air and water temperature (the water temperature in particular definitely will affect the solubility of any gas; decreases as temperature increases). So maybe we can use a published figure for CO2 concentration at atmospheric equilbrium if we correct for temperature and pressure (since the published number is typically at STP - standard temperature and pressure).

Will barometers become a standard part of planted tank hobbyists' kits??


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## BryceM

All the reference numbers are published for CO2 in distilled water. Exactly what the CO2 concentration is at equilibirum point is doesn't really matter. Why be obsessed with ppm numbers anyway? They seem to be incredibly hard obtain accurately and one person's 60 ppm is another's 20 ppm. Why not just talk about pH depression? A 1.0 pH unit drop is pretty easy to measure & compare between tanks. Ridiculously simple IMO.


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## dkfennell

I agree with guaiac_boy on this. If you look at hoppy's post, you will see that he is quite candid that the color chart he was using to measure pH in his distilled water was hard to distinguish. And if you do the calculations you will see that even a 0.1 difference on a log scale makes the result widely different.

The problems are: 1) pH tests are not reliable (at least they can't be replicated by different people using color tests; and even if digital probes are used, you have to have careful calibration); 2) you can't figure out what acids are in aquarium water, or at least you can't control for them; 3) KH can only be replicated if people "calibrate" the test with the method Tom Barr has repeatedly suggested. 

There are probably other points, but my brain gives out when I keep going round and round on this ppm of CO2 thing. 

In any event, the foregoing is enough to show that we cannot figure out the ppm of CO2 in the tank with any where near reasonable specificity. And I for one will not take any action based on someone else's statement that "so-and-so happens at x ppm CO2" or "the fish can survive even in x ppm CO2." So the question is: what is a reasonable way to describe how much CO2 I put in the tank (either to someone else or in my journal, for future reference.) And I think the answer is: enough CO2 to reduce de-gassed tank water from pH __ to pH __, as guaiac_boy has often suggested.

By the way, I think you should not only specify the pH drop, but also the beginning and ending point. If you go back through this thread and the links contained in it, you will see that under the formula shown by both MarcosB and Roger Miller, the amount of CO2 injected in the tank which results in a pH drop of 1, varies (by exponential factor) depending on the value of the starting (degassed) pH. 

Darrell


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## RTR

Air equilibrium for dissolved CO2 is *not* a hard, fixed number. It depends on the elevation above sea level and the atmospheric pressure.


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## R0bert

guaiac_boy said:


> All the reference numbers are published for CO2 in distilled water. Exactly what the CO2 concentration is at equilibirum point is doesn't really matter. Why be obsessed with ppm numbers anyway? They seem to be incredibly hard obtain accurately and one person's 60 ppm is another's 20 ppm. Why not just talk about pH depression? A 1.0 pH unit drop is pretty easy to measure & compare between tanks. Ridiculously simple IMO.


And one person's 1 ph drop could be half another person's 1 ph drop.

Using a cheap digital ph probe, my degassed tank water is ~ 8.5 and my tank is ~ 7.0. That's a one and a half ph drop and yet I still have algae issues.
Including green spot where P is high - ie a CO2 issue.

My ph tester may not be accurate, and Hoppy's may be worse, so still no one knows if the starting point is 0.5 or 3 or 4ppm. (According to the chart, 8.5 ph at 12 kh = 1.15 ppm CO2.)

I'm just guessing, but the KH might have something to do with the equilibrium value. Mine is 12. So maybe as well as describing our "drop" in ph, we shouldn't neglect to state our KHs and hence the theoretical ppm based on the old chart.


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## BryceM

Bottom line: I think we can all agree now that the exact concentration of CO2 in a person's aquarium is pretty hard to know precisely. It probably isn't that meaningful when a person states that his/her CO2 is X ppm. There are significant errors in our ability to measure it, especially if you acknowledge the possible compounding effect of measuring mulitple variables with imprecise methods. There are probably significant errors in our understanding of the interactions of CO2 in a complex solution containing a mixture of many acids, organics, and buffers.

There are many issues that come into play when trying to defeat algae - CO2 is just one, albeit _perhaps_ the most important. If there was a universal solution, someone would have figured it out a long time ago, and we'd all be following it now. There is no sense in being truly dogmatic about most anything we do since there are few things that we understand well enough.

It is perfectly reasonable to use as much CO2 as your fish can safely tollerate. Increase it until they show stress and back off a couple tenths. You need to be reasonably certain that your KH is stable if you are making decisions based on pH readings.

It seems reasonable that one could trust a relatively new, recently calibrated pH probe with more confidence than a bunch of hobby-grade KH/CO2/pH kit results. In the end, we're not trying to put a man on Mars or keep a patient alive in the ICU. This is a hobby, and there are plenty of other things to worry about too.

I'm getting the sense that the usefulness of this thread has just about run its course. If someone can find actual evidence that supports a better way to do this, please share it. I'd especially be interested in knowing if someone can accurately determine the resting equilibrium of CO2 in a buffered solution, or if they can determine if the actual KH affects the degassed CO2 concentration.


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## R0bert

guaiac_boy said:


> I'd especially be interested in knowing if someone can accurately determine the resting equilibrium of CO2 in a buffered solution, or if they can determine if the actual KH affects the degassed CO2 concentration.


Me too. It would be nice if someone who has access to quality ph and kh testers and pure water and buffers can publish a set of results for us to look at.

I agree that as part of a hobby we shouldn't get hung up on exact ppms etc, but it would be nice to have a set of standard resultsto compare with.


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## SnakeIce

This idea sounds good, however a few things need to be spelled out so the brand new planted aquarium keeper that has no history of dealing with measurements of water hardness and ph can use this.

One caveat mentioned is that your buffering/hardness needs to be at least a value. I have run into several units of measureing for this; grains of hardness, mg/L, parts per million, and any others I've forgotten. For simple use a target value range listed useing all the units of measurement that various tests may return and an amount of ___*__ per unit of volume to add to get at least the target value if your buffering falls below the target. 

*Probably would be fine to list how much to add to bring distilled/ro water to that level since higher than the target is not a problem.

A mention of both comercial products and chemical names/symbols of what can be used to raise your buffering would help as well.

the various units of measurement and not haveing conversion table for the target level of buffering has been a trouble point for me. Once this is explained, the chart in this thread would be easy to use. I think this idea with these explanations is a step towards developing a formula for success.


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## hoppycalif

guaiac_boy said:


> All the reference numbers are published for CO2 in distilled water. Exactly what the CO2 concentration is at equilibirum point is doesn't really matter. Why be obsessed with ppm numbers anyway? They seem to be incredibly hard obtain accurately and one person's 60 ppm is another's 20 ppm. Why not just talk about pH depression? A 1.0 pH unit drop is pretty easy to measure & compare between tanks. Ridiculously simple IMO.


I like this method best of all, providing we can be reasonably sure that none of us will have enough CO2 to harm the fish if we use it. When I check the CO2 I look for .8 to 1.1 drop in PH before I consider adjusting bubble rate. So far I see no gasping fish when I am at either number. And, I am reasonably sure I am at an adequate CO2 level for either number. Frankly, I doubt if I could recheck PH ten times, with the PH above 7.0 or so, and get the same number more than half the time unless I fudge my eyeballs while doing so. (Don't try fudging your eyeballs unless you are a professional!!)


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