# Re: CO2 system with/without pH controller

```> C'mon, man, it's a scientific fact that O2 and CO2 are independent.
> Chuck Gadd's experiment demonstrated this. You are wrong. Period.
> Adding CO2 does NOT reduce O2.

I am afraid it is you who are wrong. Gasses in solution are not independent
of each other. Adding CO2 (or any other gas) to a tank will in fact reduce
O2. This is expressed by Henry's law which states that the volume of a gas
dissolved is proportional to its partial pressure. Chuck's experiments must
have overlooked something, or more likely the changes were not detectable by
his measurement methods. Gasses in solution are measured in terms of their
partial pressure, and at equilibirium the partial pressure of the dissolved
gasses will equal the pressure of the gas. For example, air is (appx) 78.97%
nitrogen, 21% oxygen, and 0.03% CO2, and at sea level the atmospheric
pressure is 760 mm Hg (millimeters of mercury, a measure of pressure also
called torr). At equilibrium, water will contain these gasses in the
following levels:

N2 600.17 mm Hg
O2 159.6 mm Hg
CO2 .22 mm Hg

The amount of each gas that is dissolved in the water will depend on the
temperature, but the proportion of the gasses depends only on their relative
partial pressures. Suppose then you add some CO2 to the system so that the
air now contains 77% N2, 18% O2, and 5% CO2. The total pressure remains the
same. The water will now contain the following:

N2 585.2 mm Hg
O2 136.8 mm Hg
CO2 38.0 mm Hg

By increasing the amount of CO2 from .03 to 5% you have decreased the amount
of O2 by 15%. Adding CO2 to a tank does reduce O2, despite the proclamations
of some "experts." That being said, the important question is whether within
normal paramters for an aquarium the addition of CO2 can cause enough of an
O2 drop to affect the fishes. I do not know the answer to this one, but it
is a possibility.

FWIW, I am a professor of physiology and I do in fact understand these
things!

Peter G. Aitken

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