Something seems to defy all logic bettybaby when you state that an air stone would not work with CO2 in imparting carbon into the water because the bubbles are too big.
The CO2 needs time to allow for the carbon to mix with the water. With the slower amount of CO2 entering the water through the air stone it would need more contact time with the water. The atmospheric oxygen pumped through the air stone at greater quantities allow for the oxygen to be plentiful in the tank at night while the fish and plants still consume this commodity - rather than plants creating it during the day light hours and photosynthesis.
Otherwise I would have some oxygen starved fish in the morning.