In article ,
"Dave S" wrote:

Is there any value with pumping CO2 into an aquarium during the day and
oxygen at night?

Any value - yes, if growing lots of plants is on your menu, and you have
lots of light during the day, and you've overstocked the tank to the
point where the oxygen crashes at night. Enough value to make it
worthwhile - probably not, really. Anything beyond the simple, low-tech
yeast & sugar-water CO2 generator has never seemed the least bit
economical, and even that seemed of debatable worth, given that it's one
more thing to fuss with. If plain air won't keep the tank oxygenated,
something is direly wrong. Far better to put the money you might spend
on such a system into larger tanks with big filters (and not overcrowd
them), IMHO. DWYL.