hey,
I am not very experianced with CO2 Reactors, but from what I understand you want to get the bubbles as small as possible to have it disolve, so probably use an air stone, even better would be to put it on th intake to your filter so that the CO2 disolves in the filter
What kind of reactor are we talking?
I wouldn't think an air stone would be necessary, the sponge and things inside are there to break up the bubbles to aid diffusion.
Sam