You commented on my thread and I thought it was here for a bit... so I'll paste my response in here as well:
You'll figure it out. I think that perhaps you got talked into the whole CO2 idea as an after add on to accommodate the light intensity... at least that's what it appears to be.
Maybe you could use an opaque plexi glass to reduce the light?
Actually, You could put a lattice work over the tank to block out some of the light... might even make an interesting effect in the water. The thicker the strips, the less light gets through. You could even make it adjustable. Tape across the fixture at intervals would do until you figured out how much to block.