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RE: [APD] CO2 Levels

If a sample of water is exposed to the air for 24 hours, what will be the
level of CO2 in it at after that?

There's no real way to know without a lot more information for the following reasons:
1. It's pH dependent -- at lower pH more CO2 in the water exists as CO2 and less exists as carbonate
2. it's temperature dependent -- as the temperature goes up CO2 is less soluble in water -- you can look up Henry's law and read about this
3. it's pressure dependent -- pressure of CO2 or concentration of CO2 in air at a given pressure shifts the equilibrium. Higher CO2 pressure means higher CO2(aq)
4. there is no guarantee that after 24 hours the body of water is at equilibrium with the air, or, in other words, there is a kinetic effect. Stronger mixing of air with water will cause equilibrium to be reached sooner.

Do pH, GH, or any other factors (within a normal range)have any affect on
the CO2 level?

The solvation of CO2 in water is not a straightforward affair the way the solvation of O2 or N2 is. CO2, like SO2 and NH3 and NO3, forms new substances with water. CO2 dissolves in water to form CO2(aq), (a dissolved molecule), H+, HCO3- (bicarbonate), and CO3-2 (carbonate). If you let the water sit over an infinite period of time all of those substances come to equilibrium with the CO2 gas in the air. If the only substances you have are air and pure water at some temperature and pressure then I can tell you what the CO2 concentration is at equilibrium. That doesn't describe an aquarium very well though.

Probably all those ranges you are reading are somewhere in the ball park. If you want a better idea, measure the KH and the pH and use the charts. I haven't done the math but assume that the charts are all based on a temp of about 25C.

And, in a well-planted aquarium well stocked with fish, without CO2
injection, how much would that level
rise due to normal biological factors?

Don't know but the charts will still hold.


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