Another point to consider is the output end of the equation. I have an inline diffuser on my 120, and I don't believe that the CO2 is staying in the water as long as I would like. I have a real hard time getting the CO2 to stay over 30 ppm, no matter what my bubble rate. Right now, I have the spraybar output under the water, aimed slightly down. This gives me some surface agitation, but not a whole lot.
I'm going to be tearing down the tank to deal with some serious BBA issues that have been plaguing me. I am trying to figure out the best way to handle the output. I'm thinking of running the spraybar vertically in the tank, instead of horizontally. Worst case, I will play with several configurations and see what the results are.
BTW, the plants are going absolutely nuts - growing and pearling like crazy. The tank looks absolutely gorgeous (to me), except for the BBA. I stopped fertilizing a couple weeks ago - every time I added ferts, the algae took off - no matter what I did. Most of the algae is under control right now, except for BBA. It's not growing "big", but it is sprouting up all over the place.
I know I'm going to end up cutting out a lot of plants, but with the algae issues I don't think I'll be offering this batch to the group. I don't think I would be doing anyone any favors by giving this stuff away.
EDIT: Sorry, went off on a tangent there. Use an inline diffuser, but make sure that the output has enough time to transfer the CO2 to the rest of the tank, instead of outgassing immediately as mine seems to do.