I have a 175 W MH, 10,000 K that is about 6" from the water surface. My acro's are about 2 - 3" under the surface, so a total of about 8 - 10" is best. If this is a new light, please adjust your corals very slowly. On for 30 min. and off for about an hour and then slowly increase from there. I woud say for about a week or two before you get too the full 10 hour light cycle. other wise the corals will bleach.
reefman@xmission.com