A single 400W Bulb will illuminate the full 36" length as long as you have the proper reflector. I have a spiderlight reflector and my lights are approx. 10" from the water's surface. I do have 2 400W'ers in a single ballast right now and I love the color the 10K usio and the XM produce together. I do not run them both on at the same time anymore though. My
Blue tort did not like all that light and has yet to fully recover..
All my other corals did just fine. with the 800W configuration, my purple monster was growing fast but lightened up to a light purple. Bill Bramucci was over to my place about 1 month ago and didn't even recognize it.
I am playing around with 4 actinics to see if the colors come out anymore with more supplementation.
Right now, my lighting scheme is Ushio 1 day for 10 hrs, XM next day for 10 hours...alternating days.
HTH,
Greg