Aqua Ultraviolet - Excellent Quality, excellent customer service. I just hooked up their SL 200 and in 5 days my 4 month battle of Cyno, Dino's and GHA are all gone. One of the best pieces of equipment I've added to my reef ever. I should have done this long ago. The big question will be, what are you trying to do, Clarify water? or Sterilize water. It takes a much larger U.V. to actually sterilize water. The below website has some good info on sizing. ( some people will disagree with the microwattts per cm2 to kill algae, bacteria, & Protozoans. the Protozoan being the bug that will require the most power and dwell time, along with a few different formulas for figuring the watts needed. THis is where the debate starts ( Protozoans )
http://www.aquaultraviolet.com/products/uvsterilizers/classic