I use about twice the light as inches. For instance my screen is 6x6=36 inches and run 2 44Watt CFL Curly "Q" bulbs. This is what works for me, SM as a pretty good guide to this in other posts. Like here, http://algaescrubber.net/forums/show...ing-guidelines . Also Floyd has a great instructional here!!! http://algaescrubber.net/forums/show...cs-The-Summary