As Dillon mentioned I run a 4' on a 6' tank and find the spread both ways to be adequate. While I wouldn't say they make a huge difference when the t5s are all on (some more pop but not like a night and day difference), they definately make a big, pleasing, pop when they are on alone (to the point of finding corals that are dropped, little dna frags that have blown around, all sorts of stuff that I never saw). I'd do it again (and probably will on my other tanks).