Well, it'll all depend on your corals and personal preference. The closer you place the unit to the surface of the water the more often you'll have to clean it off.I've read through a bunch of pages on here, but didn't find any recommendations for how high off the water is recommended for t5 fixtures.
Any guidelines for how high?
Let's say... for instance... 24" 6 bulb ATI dimmable sunpower on a 24" deep cube tank?
Personally, I place my T5 unit 4-6" off of of the rim of the aquarium. I like to have the unit as close as possible, but it basically comes down to personal preference. I'd say 8" would be the max height. Anymore than that and you'll start to lose intensity.