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RE: CO2 and O2

Ok Folks lets back up a bit.

1) Please do not confuse the active transport of O2 by hemoglobin and
the dissolving of O2 in water, O2 does not actually dissolve in the same
manner that salt or CO2 dissolves.  O2 is present in water because of
atmospheric pressure (i.e it follows Henry's Law for you Chemists).  O2
is present in the blood because of the ability of the hemoglobin
molecule to bond an O2 molecule.  

2) In nature CO2 does not follow Henry's Law because of the way it
combines with any cations in natural waters.  This means that CO2 is
dissolved more like a salt then a gas in our tanks.  The dissolution and
removal of CO2 from water is involved in those pH swings people like to
ask about.  As an aside, CO2 does not typically attach to hemoglobin in
your blood.  It dissolves into your blood plasm the same way it
dissolves into water.   

3) When CO2 does not follow Henry's Law it does not affect the
saturation level of O2 which does follow Henry's Law.  If you injected
N2 (a gas which follows Henry's Law) into your tank you could force out
the O2 but not by injecting CO2 at the levels that we can.  

4) SAV's store most of the night's CO2 within their lacramal tissues and
actually store a great deal of the day's production of O2 as well. 
Algal blooms and die-offs are what cause the O2 crashes that cause fish
kills in lakes not SAV's.  SAV's are unlikly to remove enough O2 from
your tank at night to cause distress to your fish.  It is the bacteria
in your tank consuming any decaying matter that threatens your fish.  If
you really think that O2 deprevation is a problem supply a little
surface agitation, a small aquaclear filter would do the job.  

All that said, it is very unlikely that the CO2 killed that poor guy's
fish.  I hope he figures out what it was.

Sean Murphy
Fisheries Biologist