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CO2 and O2

> 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.
    The major mechanisim, which no one has mentioned, as far as I can tell,
is the plants themselves. The fish are producing CO2 and using up O2 all the
time, while the plants are using CO2 and producing O2 in the daytime but at
night the photosynthesis process stops and the plants consume O2 and release
CO2, so at nite there is no O2 production and O2 comes from gas exchange at
the surface or gas injection only. So after dark plants become part of the
"problem" and a tank that isn't over-stocked with fish could still run low
on O2. If the spaace between the water surface and the glass has filled with
CO2 (heavier than air) there could be no source of O2 available. (Tanks,
usually salt water, with refugiums or algy/plant based filters often light
these seperate areas at night and the major tank during the day so that
photosynthesis is always happening somewhere. So, as long as there is a
reasonable source of O2 at night everything should be fine, but the frequent
practice of avoiding surface agitation to prevent CO2 loss means that gas
exchange at the surface is often not as big a factor in our planted tanks as
it is in many tanks with various filters running all the time.