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Re: CO2 and anabantoids
This is an interesting topic, but I would be shocked to learn there was
actually any danger from this. Consider:
-- CO2 poisoning would happen from the amount of CO2 dissolved in the
water long before the CO2 in the air got high enough to cause problems,
unless the labyrinth organ is much more CO2-sensitive than lungs.
Since the water surface is in equilibrium with the air immediately above
it, if the CO2 concentration right above the water got to even 3% (*well*
below toxicity for humans), the water would have 40ppm dissolved CO2.
This also suggests that even the tightest-fitting lid can't be all *that*
tight; you're constantly bubbling CO2 in, but the above-tank air is
equilibrating with the outside air fast enough that the CO2 concentration
never gets above about 1%.
-- No layer of heavy CO2 will form at the surface.
Hoa Nguyen is absolutely correct in saying that it would be a gradient,
not a layer, that the magnitude of the gradient depends on temperature,
and that it would be very small at any reasonable temperatures. The
potential energy gain from moving a molecule of CO2 down a few cm and
displacing a lighter molecule of N2 or O2 is about .00001 times the
ambient thermal energy, which leads to a gradient of well less than a
thousandth of a percent. The entropy gained from mixing the gases is
much more important here than the potential released by lowering the
heavier molecule, since the gravitational force difference is so tiny.
Also, these molecules are not just settling into layers -- they're
whizzing around at 10^5 meters per second, bouncing off the lid and
water surfaces thousands of times per second.
-- The cave in Italy sounds apocryphal to me.
I searched AltaVista for La Grotte de Cane, Grotte "carbon dioxide",
Grotte "CO2", and +cave +"carbon dioxide" and found nothing. Not
conclusive, but coupled with my back-of-the-envelope calculations, it
makes me awfully suspicious. Remember, if the CO2 in the air were high
enough to kill a dog, it would be several hundred ppm in the water,
killing whatever was in the water producing the CO2. And for what it's
worth, I found a few web pages that studied CO2 concentrations in
caves (they use it to estimate plant and animal concentration in the
water) and none said what height they measured at -- very poor science
if there were indeed a measurable gradient.
Another myth I keep expecting to hear is that heavy radon settles in
your basement. Radon is in your basement not because it's heavier, but
because the source of radon is the decay of uranium in the soil around
the house. Migration out of the basement is slow because basements
often have well-sealed doors to minimize heat loss from the upstairs,
and toxicity is millions (billions?) of times higher than CO2, so as
the soil continually pumps radon out, enough can accumulate in the
basement to be dangerous.