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Re: CO2 and O2

> 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.
> 
Too much CO2 will also do this. I've seen it in low fish loaded tanks and
high fish loaded tanks. I only have seen it on HP disc set ups though with
low circulation. The other parameters where fine, NO3, Fe, P, lighting etc
but the CO2 was very high. They all complain about the same thing each time
also. Too much or too little and nothing just right. Some folks have no
problems with HP set ups. Some do. I never did myself but have seen too many
that have. Needle valves should take care of the rate issue and solve the
problem to large degree therefore and this has been the case with the tanks
I've changed over from a disc to a low pressure method and/or needle valve.
The currents were not changed much.
I would like to break out the DO meter I have and see about this further
since it always comes up every so often to see if it's the low O2 or the
CO2. I bet it's the CO2 is too high and not the low O2 levels. Both are
easily tested for. It'll be a while till I get too it. But it should show
some results about what is doing what and why.
Regards, 
Tom Barr