# [Wet Thumb Forum]-DIY CO2: Evening pH swings



## Stratified (Jul 23, 2003)

How much is the pH supposed to swing at night when the plants reverse their CO2/O2 cycle?

I recently began injecting CO2 in my 55 gal. tank. It wasn't going down at all until I found that my "slate" was bringing hardness/pH up. When I removed it the pH dropped dramatically from 7.4 to 6.8 over 48 hours.

When I first removed the slate I tested pH every hour until it seemed to stabilize at 7.0. When I awoke late that evening and check it again it was at 6.8 and the fish were swimming with a "list". 

I loosened the cap on the CO2 generator to prevent additional stress. When I awoke the next morning it was back to 7.0. I tightened the cap again and by the time I got home from work it was back down to about 6.8.

It seems to have stabilized at 6.8 now but I'm worried how it might fluctuate this evening again. How much is too much?

The plants are giving off O2 bubbles almost like an aerator. The fish seem okay now. Should I shut the CO2 off again tonight? I know that other articles have warned against this, but if it keeps swinging that sharply, won't this hurt the fish?

--Cal Webster

Here to serve...


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## Stratified (Jul 23, 2003)

How much is the pH supposed to swing at night when the plants reverse their CO2/O2 cycle?

I recently began injecting CO2 in my 55 gal. tank. It wasn't going down at all until I found that my "slate" was bringing hardness/pH up. When I removed it the pH dropped dramatically from 7.4 to 6.8 over 48 hours.

When I first removed the slate I tested pH every hour until it seemed to stabilize at 7.0. When I awoke late that evening and check it again it was at 6.8 and the fish were swimming with a "list". 

I loosened the cap on the CO2 generator to prevent additional stress. When I awoke the next morning it was back to 7.0. I tightened the cap again and by the time I got home from work it was back down to about 6.8.

It seems to have stabilized at 6.8 now but I'm worried how it might fluctuate this evening again. How much is too much?

The plants are giving off O2 bubbles almost like an aerator. The fish seem okay now. Should I shut the CO2 off again tonight? I know that other articles have warned against this, but if it keeps swinging that sharply, won't this hurt the fish?

--Cal Webster

Here to serve...


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## Rex Grigg (Jan 22, 2004)

I doubt it will change much lower than that. Getting a .2 point drop with DIY on a 55 gallon is pretty good.

Fish can take larger pH swings than this and much larger swings are quite common in the wild. What kills fish is a sudden change in the TDS of the water, or just to be simple call it hardness.
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## Dr.T. (Jul 23, 2003)

I run DIY CO2 in my 65 and 29 gallon tanks, and I run at a pH of 7.0 during the day. If I let the CO2 run all night (lights off), I have a pH of 6.7 or 6.8 in the morning. I've also tried turning the CO2 off at night. The following morning, the pH was around 7.4...

In my experience, then, turning the CO2 off causes a larger pH shift than if you leave it on overnight. An interesting point a member brought to my attention was that because pH and H3O+ concentration are log. related, a 3 or 4 point change GREATER than 7.0 is a larger change in H3O+ than a 3 or 4 point change LESS than 7.0

for H3O+, 7.4 - 7.0 >> 7.0 - 6.7 (or 6.6 to make it even).

hope this helps!

Jeff (Dr.T.)


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## perrush (Feb 24, 2003)

@ Dr T :

the reason why your pH is higher in the morning than in the evening without CO2 being injected is :

1) your losing CO2 due to watermovements
or
2) it's not completely dark during night in your aqua

almost no organisms consumes CO2 during dark periodes, almost all organism in our aqua produces CO2 during the night, so the pH in the morming should be lower than in the evening.

and ...

low pH -> more H+
high pH -> less H+
again









--
English isn't my native language, but I guess you already noticed that ))
--

Perrush


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## Dr.T. (Jul 23, 2003)

Got it - had the math backwards - see other thread, also. Thanks!

Jeff (Dr.T.)


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## imported_George (Mar 28, 2003)

I have recently made the decision to switch to a pressurized CO2 system...55 gal. I have been checking out regulators with and without a solenoid....cost wise. I had made up my mind to go without a solenoid and run the CO2 24/7.

The current discussion concerning the pH fluctuations that result, depending on whether the CO2 runs 24 hrs. a day or not, makes me wonder about the solenoid issue again. If money was not an issue, I guess, I could just purchase a regulator with an solenoid just in case.

There is no question that the pH is going to fluctuate between day and night, one way or the other. Are there any personal experiences that may shed some light on which of the "swings"...higher vs. lower pH... may be potentially more harmful or stressful to fish. Qualification: within the normal range of pH swings reported in the forum. Apparently, the advocates of running CO2 24 hrs. have not had bad experiences. Thanks.

[This message was edited by George on Thu July 31 2003 at 03:24 PM.]


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## mandy (Jul 26, 2003)

More math to make it more confusing...

pH=-log[H3O+]
pOH=-log[OH-]
pH + pOH = 14.00

Soo...as Dr T was saying, since its a log, a swing in one direction means more shift in H3O+ concentration than a swing in the other direction on the pH scale. But its proportionally related to the pOH scale. (Use of pH is just more common.) So that smaller shift in pH means a bigger shift on the other side - in pOH.

Glad you guys are talking about this stuff...I just made the switch to live plants on my little 10G - my first planted tank. Thinking of doing the DIY CO2 thing. My question is this: if my KH and GH are on the high side, will that mean that my pH is less likely to make drastic shifts if I add CO2. GH and KH can alter buffering capacity, right? So can I expect more minor shifts in pH?


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## perrush (Feb 24, 2003)

Only KH buffers the pH, not the GH.

The higher the KH, the more CO2 will be needed to make a certain pH change

--
English isn't my native language, but I guess you already noticed that ))
--

Perrush


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## Roger Miller (Jun 19, 2004)

This seems counter-intuitive, but the KH does not reduce the swings in pH caused by changes in CO2. Paul Sears has been pointing this out for quite a while now. You can show it with either the graphs or the formula. Since you seem to like formulae:

CO2=A*KH*10^(-pH) (A is a constant)

solve for pH

pH= log(A*KH) -log(CO2)

and take the derivative

d(pH)/d(CO2) = -1/(2.3*CO2)

no KH in this last equation.

What KH does do for you is move the point where a swing will start. If you have low KH then a one-unit drop in pH might be from 6.4 to 5.4 -- pretty harsh. If you have a better buffer then the same drop might be from 7.2 to 6.2 -- not quite as harsh.

Roger Miller

Edit: corrections were made

[This message was edited by Roger Miller on Sun August 03 2003 at 07:29 PM.]


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## mandy (Jul 26, 2003)

Thanks Roger....I didn't quite understand the relationship. As for formulae...hehe, I don't really like them. I just took an organic chem I final a few days ago - couldn't get them out of my head right now if I tried. (Kinda nice though, too, to see some actual practical application...good inspiration since orgo II starts in a couple of weeks!)

So in light of all this...how would I estimate how much CO2 to add?


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## perrush (Feb 24, 2003)

@ Roger :

there is a misunderstanding between us.

You mean :
A rise in [CO2] from 3.0 to 7.4 mg/L will always trigger a pH drop of 0.4 point, no wether what the KH is.
That's completely correct.

I mean :
Practical people have a set pH in there aquarium. They try to keep pH a 6.8 for instance.

If they have a KH = 5 they will have 23 mg/L CO2 in the water. At night the plants produce CO2 ... Let's assume that they are able to let the [CO2] rise to 37 mg/L, (that's an increase of 14 mg/L) the pH will drop 0.2 points to 6.6

Now for one reason the KH in that aquarium drops to 2. They maintain the same pH (by controller), so they only have 9.4 mg/L CO2 in the water. The plants in that aquarium are able to produce 14 mg/L CO2 during night, so the [CO2] in the morning will be 23.4 mg/L.
That 23.4 mg/L with a KH of 2 means a pH of 6.4, or a 0.4 drop instead of the 0.2 drop at KH=5

I know that what you said is theoretical correct, but people don't start from CO2 levels, they start from their pH, that's why they say a high KH is more stable.

maybe we should change that sentence to :
Water with a higher KH is more stable AT A GIVEN pH.

(will include a gif to make it more clear)

edit : http://users.pandora.be/perrush/KH-CO2.JPG

--
English isn't my native language, but I guess you already noticed that ))
--

Perrush

[This message was edited by perrush on Sun August 03 2003 at 10:30 PM.]


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## Rex Grigg (Jan 22, 2004)

> quote:
> 
> I mean :
> Practical people have a set pH in there aquarium. They try to keep pH a 6.8 for instance.


 I think that should read ANAL people have a set pH. Many of us don't run pH monitors and controllers. Our pH varies a bit during the day/night cycle just like it does in nature.

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## perrush (Feb 24, 2003)

> quote:
> 
> Our pH varies a bit during the day/night cycle just like it does in nature.


rex, wake up, if you think you're running a natural system.

cull 90% of the plants and fish load, get a 5 times as big aquarium as you have now, and even then you won't come close to a natural enviroment. So don't begin about natural, there is nothing natural in any of our aqua's.

and my phrase should have been : "practically seen, people set ..." and not "practical people set ..."
so try to modify your ANAL qoute too, mister marine.

--
English isn't my native language, but I guess you already noticed that ))
--

Perrush


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## Rex Grigg (Jan 22, 2004)

I stand by my comment. Too many people in the hobby want perfection. Perfect temperature, perfect pH, perfect kH and so on. Life is not perfect. Things change.

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## Dr.T. (Jul 23, 2003)

Hey now - play nice...

Personally, I run DIY CO2 in my tanks, so I don't have the luxury of the ability to control the bps or turn off the CO2 at night. For most applications - so far, I find - this non-technical approach works fine.

My tanks (pH 7.0 during the day - KH 6)drop to around 6.8 by morning if I let my CO2 run all night. If I shut it off at night, I get a jump to about 7.4. From conversations I've been having on this site, I just leave mine on.

Simple.

Jeff (Dr.T.)

Tank info in profile


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