Actually, you need them up higher than that. I just bought some myself, and to get the proper spread of light and avoid shadows, the bulb should be 16" off the water. That means the base of the reflector should be 12" off the water.
Tonight I was doing some PAR measurements with mine to see how strong they are, and let me tell you it is easy to scorch your corals with these reflectors if you aren't careful and are installing them over an established reef.
For years, I've used Spider reflectors with 250w ReefLux 10,000K bulbs. My huge Toadstool Leather was receiving 162 PAR from that bulb. I just installed a brand new bulb in that reflector, with the bubl 18" from the water. Leaving the bulb burning for 30 minutes, I took a measurement and it was 444 PAR at the exact same spot (12" submerged). I'll be posting all my findings soon, but figured I'd give you some preliminaries.
These reflectors produce so much more light that you don't need to run them as long. When I first installed them, I ran each bulb 4 hours a day for a full week. For the next week, I increased the light by .5 hour per bulb. By the third week, 5 hours per day per bulb. And after a month, I'm up to 6 hours a day. My corals are growing, nothing bleached out and everything looks great. In addition, I have more room to work above my tank because the lights aren't in my way and the water doesn't heat up because the space between the bulbs and the water is greater than my old lights. Running the lights less means pay less electricity too.
