I have the corallife 384w power compact unit (72") 125g tank, that sits on stands right on the tank rim. I would say approximately 4-5 inches from the top of the water. My tank is 23 inches deep with a 2-3 inch sand bed. I have zoas, mushrooms, paly's, sponges, various leathers, kenya trees, colt coral, a very large RBTA, and LPS corals (torch, candycane, frogspawn). Halimeda macroalgae. It is very possible to have a jambed packed reef with pc's just remember to clean the lens of the light every 2-3 days if it sits on the tank for the optimum lighting.
I am very pleased with this unit. I change the bulbs every 11-12 months. I am looking into getting the pro unit with halides, but only because I want a clam in the worst way.