The air bubbles don't hurt your method of CO2 disbursement, they mearly gas off the CO2 that you are working so hard to inject. Anything that disturbs the water surface (airstones, splashing from a HOB filter when the water is too low, biowheels, etc) cause the dissolved gasses to reach equilibrium faster. Since the equilibrium for CO2 is 3ppm and you're trying to reach 30ppm, this is a bad.