On Mon, 20 Oct 2003 15:29:24 UTC, "gizmo" wrote:

...lso think it is very important to keep the
tank aerated when light are off in order to prevent big PH drops when plants
also eject CO2.

This and a number of other postings seem to assume that adding CO2 pushes
out O2. This is not what they teach in chemistry classes (or, I think
thermodynamics classes). The dissolved gases don't compete for space in
the water, but tend to reach levels that depend on the amount of each gas
in the air.

Adding CO2 could displace O2 if you opened up the tank and filled the
whole room with so much CO2 that you had a reduced level of O2; but you're
not going to do this.

_Plants_ in the dark remove O2 and add CO2; but the removal of O2 is what
matters. Adding more CO2 from a tank won't push out O2 unless you're
bubbling so much through the water that it acts like a de-aerator. (And
even this won't happen, according to the widely held opinion that aeration
does not work by exchange with the air in the bubbles, but by agitating
thw water.)

If there isn't enough O2 in the water at night because the plants are
using too much, you'll need to add aeration; this will increase the O2,
and it will remove some of the CO2 that you're putting out money and
effort to pump in: not a desired effect, but an unfortunate side effect.

(I'm not saying whether or not it's really necessary to do extra aeration
at night when you have plants. Anecdotal evidence says it is. Somebody
ought to do serious experimenting some day with a good O2 test kit. I
can't, because my tank is massively aerated by the trickle filter, 24/7 --
and I use up a lot of CO2 that way, but it's not all that expensive.)


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