Whether you can do that depends on what lighting you have. If you have high light, you need to have the CO2 bubble rate set so the concentration of CO2 in the water is pretty close to the maximum the fish can live with. But, if you use that same amount of CO2 at night, with the lights off, so the plants will not be using any CO2, the concentration may increase above what the fish can live with. With lower light intensity, you don't need as high a bubble rate, so you can leave the CO2 running 24 hours, and just be wasting some CO2, but not harming the fish. In either case you need to have well oxygenated water, so the fish's tolerance for CO2 is high.