Using the filter as a diffuser will work, but the agitation of a sponge filter will off-gas a lot of your CO2
. You'll still need an air pump. If you've got enough CO2
pumping into the tank that you can run a sponge filter, you'll poison the fish.
Airstones can also be used, but they put out bubbles that are larger than optimal.
runs day and night. A check valve only stops water from siphoning down the tube to the bottle. Pressurized setups use a solenoid valve to shut off the CO2