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CO2 and O2

At 08:36 PM 1/12/2000 -0500, after a discussion of CO2 and O2 in water,
Sherman asked:
>Does anyone know for sure what's actually happening here?  

What makes this a little complex is that there are a couple of models that
apply.   One is a model of water in equilibrium with gases over the tank.
In this case you have gas pressures proportional to their relative
abundances in the gas mixture, and adding up to 760mm of Hg, more or less.
 That's the gas part.   Then if you pump that gas thru the water (like by
using an airstone), you will get CO2 in equilibrium with the partial
pressure of CO2 in the gas.   Similarly for O2.   If you pump pure CO2 in
one stone and pure O2 in another stone, you can get water whose equilibrium
gas pressures total more than 760mm Hg.  So it all depends on what you
measure and the model you are using.

The bottom line to suffocation is that if you run a CO2 stone so that a lot
of CO2 is released in gas form at the surface (excess bubbles that don't
dissolve).   Then the CO2 can drive off (physically) O2 and N2 from the
space between the water surface and the cover glass.  The gas over the
water can become all CO2 (assuming limited ability of air to intermix with
the gas at the surface).  If this happens, and if no airstone or other
aeration device is operating, there will be no source of O2 in the water.
The fish will die from anoxia.

What prevents this under normal circumstances is that the gas over the
water (and under the cover glass) is pretty much like the general
atmosphere (about 20% O2).  This is enough unless there are too many fish
in the tank, then O2 must be encouraged to enter the water by using an
airstone or the venturi inlet on a powerhead or ....  I suppose in extreme
cases you might even hook an O2 tank to a diffusor like we do with CO2.
This should permit even higher fish loads before the O2 runs out.

The problem comes when there is no O2 injection and there is a mechanism to
cause large amounts of CO2 to accumulate between the water surface and the
cover glass.   Then the O2 can be physically driven out of that area and
anoxia can result.

I hope you find this clear.
Dave Gomberg, San Francisco            mailto:gomberg at wcf_com
NEW Planted Aquaria Magazine:        http://www.wcf.com/pam
My aquarium plant supply store:      http://www.wcf.com/store