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CO2 Suffocating fish

I tend to agree with Gregg Tong, that concentrated co2 from the generator was
totally to blame for the death of the fish.  The gap between the glass and
the water is not particularly significant. We may be misunderstanding exactly
what we are talking about here! :-)  The size of the gap AROUND the cover
glass, or AT THE REAR OF THE COVER GLASS - The HORIZONTAL gap - is the gap
that concerns me. CO2, due to Brownian Motion, will rapidly dissipate through
a one half inch horizontal gap, so you would have to have a really LARGE
supply of CO2 to effectively "drive out" or "displace" the air. It can be
remarkably difficult to keep a layer of concentrated CO2 at the bottom of ANY
container, even though it is quite significantly heavier than air. It manages
to disperse, through Brownian Motion, quite quickly.

BUT - if the cover glass was a significantly tight seal HORIZONTALLY, you
could fairly effectively displace the air with CO2, at least to the point
that asphyxiation could occur.  Water, at saturation, contains .02 percent
O2, compared to about 20 percent O2 in air. A half inch gap between the cover
glass and the water surface is small enough under such circumstances that the
dissolved air vs. dissolved CO2 could start a real avalance effect, i.e., a
sensitive fish or two die, the body slime starts initial decomposition, the
O2 drops more from the decomposition, more fish die, more problems, things
get worse and worse quite rapidly, and the end result is an entire tank of
dead fish!  (BTW, in such cases, only air breathers, such as the Gouramies
are surviving.)

I prefer no cover glasses, but the evaporation rate is pretty high,
especially in the winter. 

ANYWAY, that is a LOT of CO2 to kill the fish!  The REACTOR design, the
simple "upside down cup" is not the problem.  The problem would have to be a
LOT of CO2, a small air gap between water and cover glass, and a very tight
fitting cover.  I suspect a pretty heavy fish load as well, but the size of
the fish load was not given. 

Hope this is helpful.