One fear people have when injecting CO2 into a planted tank is that too much CO2 will mean less O2. The reality is, CO2 does not DISPLACE the dissolved O2 in the tank. There is a limit to what levels of CO2 you want, but its the possibility of sharp pH swings.
I've not had an airpump in any tank since the early 80s, with one exception: I breed aquatic worms to feed some of my fish and the airpump in that case is just for water movement. No other way to circulate water works in that application.
My fish tanks use sponge filters with submerged powerheads. The output is directed toward the surface to make a small ripple. You can do the same with the HOB type filters as long as you keep the water level high so that there is no splashing.
