I have (2) 250 watt 14k Hami's and (2) 110 watt Actinic VHO's on my 90 (24" deep) and sometimes I think its too much. My lights are like 6" off the water and the top of my rockwork is about 6 inches below the water. About half of the corals I put on the very top will slowly bleach out and lighten up for about the first six months. Granted the growth on them is still phenominal, but they do lighten up. I also run a 10 hour photoperiod. After 6 months, they seem adapted to it, color up very brightly and still grow very quick. I'm thinking of redoing my rock work so its much lower in the tank. I can put SPS on the bottom of my tank and they still get great color and great growth. The bottom is starboard. I think 500 watts would be too much for anything less than a 90. A 75 is the same as 90, just 18" of depth versus 24". I think that that would actually be too shallow for a 75 unless you had very minimal rockwork. I would suggest going with 175's would be better, I think they make double ended 150's, from what I've heard the double ended work very well.
