Did you try mounting the lights at different heights? I have a similar sized tank, and am investigating LED lights for it. I initially purchased 5 par38 LED bulbs to play with, but need to have them mounted a 1/2" above the surface to eliminate the blue/yellow "disco ball" effect. Needless to say, the par38 idea won't work. It looks like I have the same amount of room in my canopy as you do.
Awesome system!