Wiring three fluorescent tubes

Can anyone help me figure out how to install a third tube in my 
canopy? I presently have two 30W T12's (Hitron full-spectrum) installed over a 33 gallon tank with 

I've been quite pleased with the results, but of course, nothing more 
demanding (rotala macranda, for example) is really thriving. 120W 
seems like it might be too much, especially since there is only about 
2" clearance between the tubes and the surface of the water. I'm 
concerned about heat, algae bloom, etc. So I've decided to try three 
tubes for 90W.

I got a two-bulb electronic ballast, and wired in the water-resistant 
endcaps,  but the single tube burns much less brightly than the two presently installed. The fellow at the 
lighting store says this is normal. What can I do to get a single 30W 
T12 burning at full brightness? Alternatively, would 4 30W bulbs in a 
fairly low canopy cause the problems I think they might?

Thanks for your help.

Rev. Michael A. Rodgers         When you stop to think . . .
Benediction Lutheran Church
Tsawwassen, British Columbia    Don't forget to start again.
michael_rodgers at mindlink_bc.ca