# Accuracy of ph/kh relation for CO2...



## Bert H (Mar 2, 2004)

Just wondering what factors can affect this, other than adding buffers. I just checked my pH on one of my tanks, a 10gal with ahs 2x13 light kit, pressurized CO2 and it read 6.3. With my kh of 9.5 that would give me a CO2concetration of about 140ppm! But no fish (Endlers and an otto along with red cherries) are in any apparent stress or gasping, etc. Except for a little gs, I have no algae in that tank, and growth in there is fairly slow compared to the other tanks which have higher light. There is no driftwood in that tank either, only flourite gravel. 

My other tanks have a pH reading of about 6.9, which translates to near 30ppm CO2. I have access to pH std solutions (7.0) from the lab where I work, and have tested the kits to know what the color corresponds to. So I don't think it's the kit. But I find it hard to believe that 140ppm of CO2 isn't causing havoc with any livestock! 

Anyone care to comment? I would appreciate any thoughts on the subject. I would say the moral to the story would be to jack up the CO2 in all the other tanks?


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## MatPat (Mar 22, 2004)

I wish I could find an answer to this one also Bert! 

It seems our water around here has something in it that throws off the pH/KH relationship. I have had over 200ppm of CO2 in my tank with Apistos and Discus and no apparent stress. The pH is checked with a Hanna 98129 meter (+/- .01) and the KH with a LaMotte kit (+/- 4ppm). If I remember correctly, my KH was 4.5 dKH and my pH was 5.78!

I now boost my KH by adding baking soda to up my KH between 6.5-7.5 dKH. I know there is a thread on this at the Barr Report but the links are over my head  I would love to get a grasp on this! I still have not been able to induce stress in the fish, but new additions definately have some problems with acclimating


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## czado (May 26, 2005)

Bert, sounds like the 10 just needs the needle valve dialed back.

What is your guy's rested tap pH and KH values? If the numbers don't make sense (~3ppm CO2) it is likely the derived CO2 value from pH and KH after injection won't either. If so, your looking at pH drop and plant and fish health follows Tom Barr's method of tests meaning little. Only my basic understanding.

Joe


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## MatPat (Mar 22, 2004)

Resting pH is 7.6 and the KH stays steady at 4.5 

In the spring and winter I have a higher GH and KH by about 1 degree but the pH has been stable at 7.6 in all of my resting tests. 

It's been a while since I've checked it so I guess I had better set a glass of water on top of the tank so I can test it tomorrow night  Maybe something has changed recently...


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## Bert H (Mar 2, 2004)

I have well water, and the kh is the same out of the tap as it is in the tanks, 9 -10. The pH out of the tap is around 8, which makes CO2 right at 3ppm. I don't think my water parameters change. Basically here in my neck of the woods, home wells are tapped right into the Floridan aquifer which is the source of all our springs in the area.


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## Laith (Sep 4, 2004)

I remember reading a post about someone who managed to test the individual "components" of the KH of their water and found that the component that is used in the KH/CO2/pH relationship (I think it was the bicarbonates) was very low in his water. So even if his KH was measuring let's say 8, the part that influenced the CO2 relationship was actually only 3. Hence insanely high apparent levels of CO2.

Sort of like the fact that even if your GH is let's say 6, you can still have very little Magnesium, depending on your water source.

I remember asking for more details on the importance of the bicarbonate relationship and how to specifically measure it. Don't think I got any more info...

Anyone hear of this?


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## Bert H (Mar 2, 2004)

I do remember a post to that effect, but don't recall any conclusions from it. FWIW, my kh/gh is mostly brom CaCO3, limestone aquifers of N. Florida. I add external Mg to all tanks.


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## MatPat (Mar 22, 2004)

Here are the two links Tom posted on his site about KH and Alkalinity:

http://www.advancedaquarist.com/issues/feb2002/chemistry.htm

http://or.water.usgs.gov/alk/methods.html#ctc1

I just wish I knew which minerals can contribute to alkalinity besides carbonates and bicarbonates and borate. Do sulfides and hydroxides contribute to Alkilinity? There is plenty of limestone here in Ohio also but my Ca and Mg are pretty good according to my tests and the Dayton Water Quality Report.


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## sllo (Sep 29, 2004)

I have a 29G tank. My KH is at 3.5d and my Ph at 6.1-6.0 measured using milwakee Ph51. I am adding so much CO2 that I cannot count the bubbles, in fact there is so many bubbles that by mid day there us a lot of CO2 build up in the reactor. No matter how much CO2 I add the PH does not drop bellow 6.0 and fish and shrimps do not show any kind of stress. I tried 2 different reactor an external and an internal and the results are the same. I have GSA problem. PO4 is around 5ppm which eliminates the PO4 problem with GSA so the only other problem remaining is the CO2.
Is it possible that the water is so oversaturated by CO2 that cannot disolve more?
I am really desperate about this problem. I dont know what else to do.
Is temperature affect the rate of disolving the CO2 in the water? My temp is 28C because I keep discus.


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## dennis (Mar 1, 2004)

sllo, sounds to me like your problem is an inefficient CO2 reactor. For whatever reason youeither do not have enough flow, to much flow, not enough turbulance, etc in your reactor to dissolve more CO2. Changing the angle of the reactor, location of the CO2 input, number of bioballs, etc will help you the most.

Matt and Bert, if most of your kH is comming from CaCO3 and not from HCO3, then it appears that is what is affecting your readings. Limestone is CaCO3 hence your harder Gh and good Ca/Mg levels. If memory serves, the Kh/pH/CO2 relationship is based on the amount of free H+ ions in the water. IF i am remembering correctly, your readings will be thrown of my having a lower source for the H in the beginning(HCO3 versus CaCO3)

Course, I could be, and probably am, completely wrong


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## Bert H (Mar 2, 2004)

sllo, I agree with what Dennis says. I had a 29 and with a rate of 1.5 - 2 bubbles per second was getting readings in the 40-50 range for CO2. What is the length and width of your reactor? How do you have it plumbed? As Dennis says, the angle has been reported to have an effect as well. 

I assume you have the CO2 feeding in from the top of the reactor, as well as the water inflow - from the top, flowing out the bottom into your tank? If it were me, I'd build a larger reactor and see how that goes.

Dennis, in regards to the numbers, I have come to the conclusion that as long as I maintain CO2 levels between 30-50, plants do well, fish aren't stressed and (knocks on wood) I have no problems. I have a feeling that our estimations of CO2 contents may well under-estimate values overall.


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## sllo (Sep 29, 2004)

I built this reactor http://www.aquaticplantcentral.com/forumapc/attachment.php?attachmentid=471&d=1099597504
the tube of the reactor is 50cm I tried smaller and bigger, I also tried 2" and 1,5" tube. It is poered by a Fuval 304 canister. I have 3 bioballs in there. I dont see any bubbles coming out of the reactor. I tried feeding the co2 from top and from the bottom. I tried also an airstone in order to make the bubbles smaller in order to dissolve faster. The water flow is from top to bottom and the reactor is connected on the return of the canister.
I tried different angles.

I built 3 days ago this internal reactor, http://www.tsamisaquarium.com/Selides/Themata/co2c.htm
which works like aquabotanic vortex reactor.

No matter what I do the results is the same.


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## Simpte 27 (Jul 16, 2004)

I found a fairly good link that explains elements that contribute to non carbonate alkalinity.
http://www.livemeter.com/guide.php#Alkalinity
I know our water here is high in silicates.


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## sllo (Sep 29, 2004)

What I dont understand is that with Kh 3.5 and Ph 6.0 the CO2 is 120ppm. If I add 1 bps the ph is around 6.6 which makes the co2 30ppm, however GSA is still there and the plants are not pearling well.
So my problem is that I need a lot of bps in order to increase the CO2 in such levels to make the plants pearl well and kill GSA and I dont understand why!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
Why do I have to add so many bubbles. Even if there was any buffers in the water the co2 would drop below 6.6 with 1-1.5 bps.
At 1-1.5 bps the reactors are disolving the CO2 very fast.


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## dennis (Mar 1, 2004)

Sllo

Pearling plants is not always a sign of sufficient CO2 levels. Plants pearl when they are healthy and photosynthesising full bore. If you rplants do not seem to be growing then I suggest you should look to your nutrients. Quite possibly you are missing or low in something. I would check NO3, PO4, K, Mg, then traces.

I my tanks I often have an opposite issue from yours and Bert's experiences. Often, when I maintain a 30's ppm CO2 my fish become stressed and suck air from the surface. The plants grow and pearl like mad. I use a CO2 controller, calibrated and check my kH frequently. I have some wood in the tank but not enough to really affect the relationship, however, if the wood did affect the relationship it would give me a false high reading causing me to be low in CO2, not high.

I wonder what/how much PO4 is needed to start messing with our readings? I have always heard that using phosphate based buffers will throw off the kH/pH/CO2 relationship but I have never heard the levels where this starts. Maybe know that adding 2-5ppm PO4 weekly is common we are having an "unusual" i ssue?


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## BryceM (Nov 6, 2005)

I'm also becoming a little suspect of the KH/CO2 relationship. There are two factors that may influence this. Not all of the acid in the tank is coming from carbonic acid. In non-planted tanks the pH gradually moves downward over time as organic acids accumulate. Water that started at pH 7.0 with no added CO2 would read 6.7 or 6.8 after a while. I'm not sure that simply adding plants 'soaks up' all this organic acid.

The second factor (being discussed above) is the buffering capacity of the water. As I understand the test kits that most of us use, the actual chemical reaction is a simple titration. An acid is added drop by drop until the pH in the tube is low enough to overcome the buffer that was there and the indicator turns colors. The test kit doesn't 'know' if the buffer is CO3, PO4 or, or something else. The reported value is expressed as an 'equivalent carbonate' alkalinity.

As you can clearly see, I know just enough about the chemistry of this to be entirely dangerous. I look at it this way: If you look at the chart for KH = 2 it takes a pH drop of 7.1 to 6.3 (0.8 units) to go from 5 ppm to 30 ppm. At KH = 4 it takes a drop of 7.4 to 6.6 (0.8 units). At KH = 8 it takes a drop of 7.7 to 6.9 (0.8 units).

I let some water degas, read the pH, and turn up the bubbles until I've achieved a pH 0.8 units below what I read. I assume (correctly or incorrectly) that the resultant drop is entirely due to carbonic acid. I freely admit that the chart is only accurate if carbonate buffering is controlling.

If you have a KH of 8 and your degassed pH isn't around 7.7 then something else is living in the water. My water tends to read lower than it should assuming a degassed CO2 of 2 or 3 ppm and a given KH. I attribute this to other organic acids and/or inaccuracies in my KH test kit.

.... just a few random thoughts on the subject. This thread is interesting and needs more input from the true 'chemists'.


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## dennis (Mar 1, 2004)

GB,

Your comments make a good deal of sense. Normally, the way one would deal with wierd water is to do exactly what you said; let it degas and compare the resulting pH with what you rpH should be based on your Kh and a CO2 concentration of ~3ppm. This difference in pH units is how much lower or higher you should shoot for when targeting pH for CO2 levels.

I think I need to check this difference again for my tap water.....

I stillwonder when PO4 starts to play an issue?


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## Bert H (Mar 2, 2004)

As Dennis said, those are good points. It's something I haven't done, but will very soon. A cup of water is now sitting on a tabletop. Tomorrow I will check pH and see what it is, and compare to my kh and what it should be.


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## BryceM (Nov 6, 2005)

Dennis,

I've been thinking 'bout the PO4 effect on the relationship. Once upon a time I might have had a better grasp of the chemistry involved. PO4 has a pK (maximum buffering capacity) at 7.21. CO3 has its pK at 6.37. The range of most people's tanks is somewhere in this vicinity so one buffering system shouldn't be that much more powerful than the other (in an aquarium at pH 7.21 the phosphate would have its maximal effect).

Assuming they have nearly equal capacity to buffer, the impact of each should be proportional to its concentration. KH 4 (if from carbonate) corresponds to 71 ppm CO3. If you have a PO4 concentration of 5 ppm it's still roughly 14 times weaker than the CO3 buffer. I'm guessing you'd need truly colossal PO4 levels (some people do) to really mess with the relationship.

I kind of think the bigger error is the organic acid in most aquariums. This would lead people to think they have more CO2 than they really do. The 'chart' only works if the resultant carbonic acid is the only acid present.


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## dennis (Mar 1, 2004)

> I kind of think the bigger error is the organic acid in most aquariums. This would lead people to think they have more CO2 than they really do. The 'chart' only works if the resultant carbonic acid is the only acid present.


 This is true and the reason why peat/wood affect the chart; they add "foreign" acids.

Here is a copy from an old Krib post...it seems to explain it but ht emath is above me.

from: http://www.thekrib.com/Plants/CO2/kh-ph-co2.html



> A while ago, George Booth asked me to consider the behavior
> of a mixture of hardness buffer and phosphate pH buffer.
> 
> The hardness buffer is based on the carbonic acid-
> ...


Anyone care to tackle the math asssuming an aquarium with 2ppm inorganic PO4


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## FobbyBobby23 (Mar 8, 2005)

could it just be your light levels aren't that high?

i had 2x13 over my 10, and that nearly wasn't enough light for fast growth

upgraded to 1x36 watt over the 10 gallon, growth got really fast and pearling was definately more evident, even though i had same Co2 concentration/rig, only thing i changed was the lighting and my dosing schedule to match the higher light level

well that'd only explain the slow growth....not the wild co2 readings!!


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## sllo (Sep 29, 2004)

dennis said:


> Sllo
> Pearling plants is not always a sign of sufficient CO2 levels. Plants pearl when they are healthy and photosynthesising full bore. If you rplants do not seem to be growing then I suggest you should look to your nutrients. Quite possibly you are missing or low in something. I would check NO3, PO4, K, Mg, then traces.


I agree. I am using the EI method. I am pretty sure that all my macros and traces are more than enough. Lighting is 2.75WPG. When Ph is at 6.3 or higher perling is very minimal. When it drops to 6.0 then it shows a big increase. Also GSA shows low PO4 or low CO2. PO4 is not the issue, I am pretty sure, so the only thing remains is the CO2.
I built a new reactor last night, the one that Tom Barrs has on his site.
I will pumb up the co2 and see what happens. I am at a phase that I really dont care if I kill the fish. (2 Discus, 4 Rams and some neons).



guaiac_boy said:


> Dennis,
> 
> I've been thinking 'bout the PO4 effect on the relationship. Once upon a time I might have had a better grasp of the chemistry involved. PO4 has a pK (maximum buffering capacity) at 7.21. CO3 has its pK at 6.37. The range of most people's tanks is somewhere in this vicinity so one buffering system shouldn't be that much more powerful than the other (in an aquarium at pH 7.21 the phosphate would have its maximal effect).
> 
> ...


I read on barrreport another thread saying this. It is pretty reasonable. 
I also have a glass sitting with tank water. I will test it tomorrow.

I found these two discussions:
http://www.thekrib.com/Plants/CO2/co2-meter.html
http://fins.actwin.com/aquatic-plants/month.200002/msg00119.html

I dont know if these assumpions and formulas are correct.


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## sllo (Sep 29, 2004)

Using the venturi reactor with the same bubble rate I used on the other two reactors the Ph dropped by the end of the photoperiod from 6.0 to 5.7!!! Still no signs of stress on the fish.
I wonder what is going on and at which point the fish will start stressing....


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## BryceM (Nov 6, 2005)

For comparison I set out some water and tested it. My aquarium is usually at 6.60 depending on the exact bubble rate (it fluctuates a little for some reason). My 'degassed' water tested at pH 7.15. With a KH of 4.5 it should have been 7.6 or so. I'm assuming 'other' acids are accounting for the difference.

My total pH drop due to CO2 is 7.15-6.60 = 0.55. From the chart at KH 4.5 this means I probably only have about 24 or 25 ppm CO2 right now. I get this at 120 to 130 bpm injected into the intake of an Eheim filter in a 46 gal aquarium.

I gotta go turn up my CO2, bye.

- GB


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## sllo (Sep 29, 2004)

Aerated tank water is at 7.1 and kh 3.5 which gives 8.3ppm co2.
So in order to have 30ppm ph should be 6.5
However my Ph is at 5.7 that should be over 208ppm co2!!!!


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## MatPat (Mar 22, 2004)

Long overdue but last night I pulled 8 ounces of water from the tap and tested it. Here are the results:

Tap
pH - 8.51
KH - 5.59 dKH
CO2 - 1 ppm
TDS - 218

Tap aerated/rested
pH - 7.45
KH - 5.59 dKH
CO2 - 4 ppm
TDS - 218

After aerating and resting overnight the CO2 has risen by 3ppm! It appears that something is indeed strange with my water.

Here are some tank water samples given the same treatment. I don't have any rock work or driftwood in my tank to contribute any acids. I do add 1 tbsp of Sodium bicarbonate (~3dKH) to my water with the weekly water changes.

Tank
pH - 6.15
KH - 6.08
CO2 - 126 ppm
TDS - 212

Tank aerated/rested
pH - 7.67
KH - 6.08
CO2 - 4 ppm
TDS - 212

These results seem to be inline with my tap readings of 4ppm of CO2. However, I may have skewed the readings by doing a couple of RO/DI water changes this week. I was expecting some Wild Peruvian Altums Sunday night and the addition of the sodium bicarb had caused my KH to rise to 9 dKH (191ppm of CO2) To avoid stressing the Altums, I did a 25% water change Sunday afternoon with RO/DI water with 3dKH of Sodium Bicarb added. I did another 25% WC on Tuesday minus the sodium bicarb to arrive at my current reading of 6.08 dKH.

I have had my CO2 levels slightly higher in the past with no fish stress.


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## BryceM (Nov 6, 2005)

MatPat, it's possible that your tap water comes directly from a city well without ever being exposed to the atmosphere, therefore having only 1ppm CO2. An increase to 4 ppm isn't that unusual since the atmosphere contains about that amount. I will agree, though, that the large pH shift is a little hard to understand. 3ppm CO2 shouldn't make a big difference with your KH. Water companies add all sorts of things though....

Acid in a tank comes from more sources than just driftwood. Huge quantities come from the fish as they metabolize. Uneaten food also breaks down in processes that produce organic acids. I strongly suspect that most people have less CO2 than they think they do, wrongly assuming that the only acid in their aquarium is carbonic acid. I also suspect that you could go up to 100 ppm CO2 or so without harming the fish provided the pH itself doesn't get too low. Am I willing to try it in my tank? No, but others report huge CO2 levels and no apparent effects. The kidneys & gills of the fish do eventually get overwhelmed at a certain acidity depending on the species.

Now, if I could just make the algae go away.............


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## MatPat (Mar 22, 2004)

I think I may have found the answer to my high CO2/BBA problems. It seems the City of Dayton uses Calcium Hydroxide to soften the water. Here is a link to using Calcium Hydroxide as a water softener:

http://www.freedrinkingwater.com/water-education2/410-lime-soda-water.htm

Assuming the info on the site is correct and Dayton does in fact use slaked lime (the city website says they use lime), the first sentence in the fifth paragraph gives me a little hope that I am indeed low in bicarbonates. I posted about this here also:

http://www.aquaticplantcentral.com/forumapc/showthread.php?t=11833

Hopefully we can get some confirmation on the above from the chemists


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## JerseyScape (Nov 18, 2005)

I am getting the same exact thing....I kept upping the Co2 into the tank and I have a PH of 6.4 and KH of 4 which would give me 48ppm of Co2. I tested many times and retested again but everything checks out. My neon tetras, ottos and shrimp are doing just fine. I wish I knew the solution to this little dilemma.


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