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Re: oxygen versus co2

Lazarus Miskowski wrote:

> Some prominent members of the list (the names escape
> me at the moment) have long been proponents of
> increased oxygen levels in our tanks.  Generally this
> means increased ciculation.

This isn't necessarily true.

> My recent experience has
> been that fish activity level was increased by
> increasing circulation.  So I am a believer in this
> regard.
> My question has to do with maintaining co2 levels.
> High co2 and high oxygen aren't mutually exclusive,
> correct?

Their concentrations aren't directly related to each other in any way.

Circulation or aeration will usually keep the oxygen level in an
unplanted tank from falling too low for it's usual occupants.  It won't
rise much over 8 mg/l, because water reaches equilibrium with
atmospheric oxygen somewhere near 8, depending on temperature, altitude
and the composition of your home's interior air.

Photosynthesis by healthy plants in a well-lit aquarium can push the
oxygen content over 10 (that's where my old oxygen kit pegged out). 
That's a higher concentration than you can ever get from circulation
alone.  Circulation and aeration tend to keep the oxygen content close
to atmospheric equilibrium levels -- keep it *lower* than the
concentrations you would get without circulation.

Oxygen levels drop when the lights go out, the more circulation you
provide, the faster they drop.  A tank is unlikely to maintain elevated
oxygen levels all night, with or without circulation.  Just how low they
drop depends on the conditions in your tank.  The tank I used to monitor
had about 8 mg/l of oxygen before the lights came on in the morning.

People sometimes report that their fish are gasping at the surface in
the morning, implying low oxygen.  I've never seen that happen in my
tanks and I have to suspect that those tanks with gasping fish contain
more fish than I normally maintain in an aquarium and/or the plants
aren't photosynthesizing enough to supersaturate the tank.

> I can increase circulation and therefore
> increase oxygen, and at the same time increase my
> input of co2 to maintain adequate levels, correct?

This might get you a net increase in oxygen if your tank is overloaded
with fish, if you don't have very many plants, or if your plants aren't
real healthy.  It won't do your plants much good, and since the plants
are net producers of oxygen your increased circulation could end up with
an overall decrease in oxygen.

> While this may be inefficent, co2-wise, I presume it
> is healthier for the fauna.  Any comments?  Am I
> headed in the right direction?

It is inefficient -- maybe to the point of being self-defeating -- and
certainly it's something you don't need to think about much unless your
fish show symptoms of low oxygen.  If they are stressed by low oxygen
then it would probably be better for all concerned to reduce the fish

Roger Miller