Hmmm, I didn't think about putting the CO2 under the powerhead, but that would mean one less hole in the tube so less of a chance for leaks! Also, the bubbles would diffuse faster.
The only possible problem is that since the tube from the powerhead has to go in an upside down "U" curve out of the tank and into the diffusion chamber, would the bubbles possibly stall at the top of this tube before entering the diffusion chamber? I was planning on having the gravel vac part as close to the top as possible, so if bubbles got out through the bottom they would still have to go down some tube to get to the top, so that would lessen the chance of that happening. When I set it up, I'll try it with the CO2 into the powerhead first, and if that doesn't work, I'll flow it directly into the diffusion part.
Also, I need something to hold the tube going back into the tank in place so it doesn't fall or something. I think I saw some replacement suctions cup heater holders at my LFS a while back, if they are the right width, then maybe those could hold it in place? I'll see if I can go to my LFS and get some parts tonite... if not it will be Saturday when I go to my piano class since the LFS is across the street from my music school