I agree somewhat that there will be drop off at the edges. You can see it in the front left of the above photo. My opinion is that a 30" tank with a 12" fixture is on the edge of having to go up to the next size as is the 48" tank with a 24" fixture on it. I don't think you could do a 36" tank with a 12" fixture. Another thing to consider is that 16 high power LEDs are crammed together without optics so it minics a MH bulb. You could easily light a 75 with two MH bulbs. Also, maybe someone could chime in, are not the modules movable? So if I bought a 12" for my 24x24x24" cube I could move the two modules a couple of inches out to the egdes to get more coverage? I bet with the 24" over the 48" if you moved the two outside modules as far out to the edge of the fixture as possible and the two center ones out a little because of the top support divider you would get total coverage.
This sounds like something I wanna hear now. A 24" lighting a 48" would be perfect. Anyone who has these please chime in if these are movable.