The CO2 tank will only last 25-33% as long if you run it 24/7. Refills are not expensive but the extra ones will add up.
If you are injecting the CO2 into a reactor (Cerges, Rex-Grigg) that is powered by a dedicated pump or power head, then the latter could be put on a timer (on when the lights are on).
Otherwise, the chances of gassing the fish is high unless you are running a very low rate (BPS).