Wonder if a fully functioning fish take would be even better - if you get plenty of plants growing, they will feed off the water/each other in my experience. The rate of air injection would likely have to be higher than that of a normal fish tank though? I used to run a fish tank with CO2 injection to get the plants growing well. The best, cheap way of doing this was to put CO2 into the filter intake, that way big bubbles which escapes the diffuser were blended up by the impeller of the filtration.