Besides heat which we know how to remove by having a fan blow across the water (not across the lights), the other problem with having a light fixture too close to the water is the shadow at the ends of the tank. The reflector is made to give a triangle of downward shining light. A fixture must be high enough to light the top ends of the tank or else it just looks well,...strange.
My preferred method is to hang the lights. I buy inexpensive chain and secure it to the ceiling with a screw into the rafter or a molly bolt into the sheetrock. I put "S" hooks at the end. The light fixture has a short chain securely attached to it. The S hook is crimped on the short chain of the light fixture. The fixture can then be unhooked and moved up and out of the way when I am working on the tank, yet is still giving light to the tank so I can see what I'm fragging. Maybe I have a pic...yes, see the loop of cable that goes around the wood frame and the extra chain hanging down past the S hook.

Here is an end shot. As you can see, this same homemade light fixture was used on two different tanks from 2003-2007. Two 175W MH were later added to the center of the fixture below. I am using the same principle on my present tank if you would like to come see it.

Edited by Mark Peterson - January 13 2011 at 12:01am