Take a sample of tank water and set it out overnight then take a pH reading.
Take a pH reading of fresh tank water in the morning (or whenever it is that you do the old water) and if the difference is 1 point of pH then you have 30ppm of CO2
. This is the maximum safe level for FW
If it's less than that, you're fine.
Only trouble is, when there is an air stone going, you're gassing off most of the CO2
and it's not doing anything.
If you have an HOB
filter then you don't need the air stone.
If you have a canister you need to set the outlet, be it spray bar or nozzle, to cause the surface of the water to gently ripple. This will put plenty of oxygen in the water and not gas off the CO2