All 4 of my tanks are CO2
supplemented with a timer-activated pH monitor. High-light, fertilized also.
I run an airstone in the 2 with community fish -at night-, when the CO2
levels might get too high. I deliberately gas off some of the CO2
with high surface agitation from the airstone. Otherwise, I get stressed/dying fish from CO2
overload. My betta doesn't need it so I don't run it in that small cube tank.
Air stone turns off and CO2
turns on about 3 hours before my lights come on so that I have CO2
at the proper level (~30ppm) when lights ramp up.
This has worked well for me and I find that I don't go through a whole lot more CO2
than when I do not run the airstone.