# [Wet Thumb Forum]-CO2 production



## perrush (Feb 24, 2003)

Hi all,

I was wondering how many CO2 a well planted aquarium can / could / will produce during night.

You can help me by giving me your highest and lowest pH of a single day (when the light go on and off) and also your KH value.

Additional info is appreciated too. (filtering over peat or not / aireation during night or not / ...








Perrush


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## perrush (Feb 24, 2003)

Hi all,

I was wondering how many CO2 a well planted aquarium can / could / will produce during night.

You can help me by giving me your highest and lowest pH of a single day (when the light go on and off) and also your KH value.

Additional info is appreciated too. (filtering over peat or not / aireation during night or not / ...








Perrush


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## António Vitor1 (Feb 2, 2003)

a lot!
I have one powerhead that only works at night, for some increased surface turbulance at night ... 

if I didn't do something at night.. fish would die... (lack of oxigen)

I have DIY CO2, so I am not able to tell you what is my tank (fish and plants) CO2 poduction...

most of us have some sort of CO2 injection... so it's dificult to give some valid numbers... 

and... what is a well planted aquarium?
that definition is very loose.

AntÃ³nio Vitor


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## MarcinB (Apr 16, 2004)

In my tank day/night pH changes do not excess 0,3 pH unit.

Tank specifications:
-150L (40G)
-90W of fluorescent lighting
-CO2 injection through the inlet of the canister filter (60 bubbles/s)
-pretty heavy fish load
-KH 6, GH 11
-no additional powerheads etc.


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## ekim (Jan 31, 2004)

My PH swings are about 0.2 from morning to night. 0 surface movement, CO2 runs 24/7!


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## wetmanNY (Feb 1, 2003)

Our pH flexibility depends on our alkalinity.


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## Roger Miller (Jun 19, 2004)

Paul Sears has made this point over and over for a few years now. The size of the pH swing that comes from a change in CO2 -- all else being equal -- does not depend on alkalinity.

Roger Miller


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## perrush (Feb 24, 2003)

> quote:
> 
> Originally posted by Roger Miller:
> Paul Sears has made this point over and over for a few years now. The size of the pH swing that comes from a change in CO2 -- all else being equal -- does not depend on alkalinity.


It DOES depends on your KH, but only for a small part.

Your pH value has much more influence.

For example :
A CO2 production of 20 mg/L during night, will :

1) will drop your pH from 7.6 to 6.5 if the KH=2 (1.1 point change)

2) if the KH=6 that same amount of CO2 would drop your pH from 7.6 to only 6.9

3) with a KH=2 that 20 mg/L CO2 production would drop the pH from 7.0 to only 6.4 (0.6 point change)








Perrush


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## Roger Miller (Jun 19, 2004)

Perrush,

You've lost me there some place. The formula that relates CO2, KH and pH is

CO2=A*KH*10^(6-pH)

There are different values for "A" and a few slightly different forms the formula, but they are all equivalent.

pH from this formula would be

pH=6-log(CO2)-log(A*KH)

The change in pH between two different CO2 values, CO2(1) and CO2(2) is

pH(2)-pH(1)=log(CO2(1))-log(CO2(2))

The KH values and the A value fall out when you take the difference.

Perhaps what you have done in your examples is not keep "all else equal" -- particularly, the starting CO2.

If you use George Booth's chart at thekrib -- which is easier to use for this then something like the Tropical table -- and compare the change in pH from CO2 of 5 to CO2 of 25 at different KH values, this is what you get:

KH=5 CO2=5 pH=7.5 CO2=25 pH=6.8 Difference = 0.7
KH=7 CO2=5 pH=7.6 CO2=25 pH=6.9 Difference = 0.7
KH=9 CO2=5 pH=7.7 CO2=25 pH=7.0 Difference = 0.7

These readings are all rounded to the nearest value represented on the graph, but you can see that the result is constant.


Roger Miller


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## wetmanNY (Feb 1, 2003)

Like many people who have difficulty balancing a checkbook, I have an awed and immeasurable respect for all chemical equations-- especially when wielded so deftly to refute what might seem like an innocuous statement, even a cliche: *Our pH flexibility depends on our alkalinity.* Amateurishly stated, I'm sure.

Paul Sears' name is certainly one to be reckoned with. So he too is convinced that "the size of the pH swing that comes from a change in CO2... does not depend on alkalinity" is he?

I'd have thought, quite conversely, that it didn't depend on "KH" or "carbonate hardness"-- which is neither wholly carbonate nor really "hardness"-- but quite precisely upon alkalinity, or buffering.

So then alkalinity is no longer considered a measure of the "buffer" that renders pH less flexible?


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## Roger Miller (Jun 19, 2004)

In English "alkalinity" is the correct term. "KH" is an abbreviation for a German term, which I believe means the same thing. Regardlesss of which term you choose to use the test kits work the same way and measure the same property.

To be quite correct, the term that needs to go into the formula is the bicarbonate concentration. We can't measure the bicarbonate concentration directly so we use alkalinity (or KH, take your pick) as an approximation to the bicarbonate concentration. It is a potentially flawed approximation.


Roger Miller


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## MarcinB (Apr 16, 2004)

"Alkalinity refers to the capability of water to neutralize acid. This is really an expression of buffering capacity." Standard aquaristic tests measure the alkalinity not KH (a measure of the carbonate concentration in
the water), but in most cases this two terms are almost equivalent, because carbonate is usually the main source of water alkalinity. According to my knowledge the carbonate does not buffer the CO2 dependent pH changes. So in this case pH flexibility should not depend on KH but may depend on alkalinity if there are other sources of alkalinity except of carbonate, which may buffer the CO2 dependent pH swings.


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## wetmanNY (Feb 1, 2003)

That K in "KH" stands for _kalk_ the German word for "lime" I think, but the two terms really don't simply mean the same thing. Carbonate is indeed the main source of alkalinity, as MarcinB says. Whatever property the "KH" tests are titrating, the results are given "as" carbonates because they do include orthophosphate, which might reasonably go into any useful aquarium-use formula, right along with the bicarbonate concentration. Biologists can use "KH" tests in the field as a rough-and-ready approximation of alkalinity, because phosphate is vanishingly low in unpolluted natural waters, but most posters here at AquaBotanic are dosing it on a regular basis.

Borate would tilt the scales too. Wasn't someone at this forum dosing _boric acid_ on a regular basis, to keep up those boron levels? Or was that at a less-enlightened forum?

In Roger Miller's equation, CO2=A*KH*10^(6-pH) does that "A" refer to acid(s)? Is this the famous equation that dismisses all acids in the aquarium save carbonic acid? That would account for the variety of values given for "A" that Roger refers to.


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## Roger Miller (Jun 19, 2004)

The "A" in the equation includes an equilibrium constant and conversion factors to match the units used. It doesn't stand for acid. There is some disagreement over what the actual value should be, some variation because of units and I didn't care to look up all the different values.

Phosphates and borates are measured as part of the alkalinity. Phosphate has to be very high before it is a significant contributor. I posted a table to the APD some time ago that detailed the amount of alkalinity produced by a given amount of phosphate. I think I also posted the table in the old aquabotanic forum. Maybe even here. I don't recall.

I hope that people are not dosing enough boric acid to be a significant part of alkalinity. Boron -- while it is an essential plant nutrient -- is also toxic to plants. For crop plants there is a rather small difference between the minimum amount required by some plants and the maximum amount that can be added without effecting the production of other plants. If that pattern carries over to aquatic plants then we need to be very careful about dosing it.

I expect that organic acids are the most common non-carbonate contributor to alkalinity.


Roger Miller


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## wetmanNY (Feb 1, 2003)

Ah. You adepts have no idea how obscure an equation with an unidentified constant can appear to us amateurs. I suppose this "A" is a widely-recognized constant, immediately familiar to anyone better prepared than I am. Will I ever catch up?

Though I never use phosphate buffers myself, I often hear instances where the phosphate of the buffer turns out to be very significant indeed, as a hidden contributor to puzzling alkalinity values.

So Roger Miller and I are in complete agreement that dosing with _boric acid!_ is rash, to say the least! --Weekly dosing, as I remember! Personally, I'm unwilling even to add copper, for the sake of the plankton that keep my water clear...

Organic acids could easily be underestimated in these calculations. They make quite a contribution to the acid-base buffer in those planted systems where there is some peat filtrate, or leaf litter, (not to mention a spent green tea bag in the filter). I can't make out any allowance for humic acids, or any organic acids at all, in the widely followed tables, such as George Booth's, and in equations interrelating pH, alkalinity and CO2. Or are organic acids subsumed within the mysterious and _flexible constant_ "A" of Roger Miller's equation?


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## Roger Miller (Jun 19, 2004)

Any organic acid that tends to associates (gain a hydrogen ion) in the range of pH from the initial sample pH to the endpoint pH (about 4.5) of the titration will be read as part of the alkalinity. There is no way to account for its presence.

"KH" stands for karbonatharte, but with an umlaut over the last a -- something I can't type on my keyboard.

Roger Miller


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## imported_Creature (Feb 6, 2003)

Perrush,

The answers you seek seem to have created a tangent. In my pH controlled tank, I've noticed that the controller will kick-in during the first part of the night as the plants continue to release O2 they have built up in their tissues.

An increase in your KH value will increase your the buffering capacity of your water, meaning, the higher the KH, the more CO2 will be needed to get the pH you want.

With my pH controller, I count no bubbles and my pH always stays within 0.1 pH units within my set point which is 6.6 for a KH of 5.0 and I complain because my tank is fizzing with O2 produce by my plants.

I use a power head to help circulate the water and I aim it near the surface so that the current doesn't cause my plants to bend acutely. The small surface ripple is nothing compared to the surface area created by O2 bubbles breaking at the surface, so I don't worry about C02 loss or about aerating when the lights go off. Unless your tank water is stagnant (not moving) you shouldn't need to worry about lack of oxygen for the fish at night.

Filtering over peat, well, a little peat would be fine, but decaying driftwood does about the same thing. Peat extract would be even easier. If your fish don't really need it, then I wouldn't bother. The peat leacheates will also hinder some light from reaching your plant pigments.

Invest in a pH Controller (Pinpoint). No fuss, no muss, no guessing and spectacular plant growth!

The Gill Man


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## imported_Creature (Feb 6, 2003)

> quote:
> 
> Originally posted by Creature:
> Perrush,
> ...


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