I started using that method, but it doesn't raise the pH too *low* and the bubbles that get shot across the tank get annoying after a while, at least they did for me. Otherwise, it's good for getting your CO2 to about 10 ppm maybe.
Well... CO2 should never RAISE the pH. When CO2 builds up it lowers the pH because of carbonic acid. Doing it with a powerhead is a quick and easy way to diffuse it. As long as you don't have much splashing to cause the CO2 to escape, it should work well for you. Just get a KH test kit and a pH test kit so you can watch it and see how effective it is. I'm sure it's better than using a standard air stone.
I rigged it all up and it is working all fine and dandy. Not shooting bubbles anywhere and causes no surface agitation at all. Pretty nifty I say. If I notice a difference in my plants here in the next week I'll be sure to post updates.
I'm going to use a large-mouthed jar, upside down, and let the CO2 diffuse into the water that way. The main advantage is the rate of dissolution will be constant, so the PH won't change and upset my clown loaches when I put on a new bottle. Any excess will bubble to the surface and escape, and there should be quite a while's worth of reserves in there.
