Here's an excellent article on UV sterilizers.
http://www.americanaquariumproducts.com/AquariumUVSterilization.html
Especially helpful is this chart:
I really think you should go for Level One Sterilization (the most common and recommended aquarium application) instead of just Clarification.
Here's how the math comes out:
Clarification at turnover rate once every 2 hrs at 40 gph/Watt:
450 gph divided by 1 W/40 gph = 11.25 Watts So you would need a 15W UV for clarification
Level One Sterilization at turnover rate of 1.5/hr at 20 gph:
1350 gph divided by 1 W/20gph = 67.5 Watts -- you would need an 80W UV sterilizer
Keep in mind that the smaller the UV sterilizer, the slower the recommended flow rate will be. Example: A 15 W Pentair UV sterilizer has a recommended max flow rate of 240 gph,
whereas the 80 W UV sterilizer flow rate can be up to 3000 gph, and a 120 W UV up to 4500 gph.
Following the recommended flow rates, I chose UV sterilizers that would work for the flow rates I already had, i.e., I connected a 40W UV to the return of my FX-5 on my 300 gallon tank. The 25 W UV would have worked, but I would have had to get another pump just for it. On our big set up, I'm connecting my 120 Watt UVs to my Barracuda Golds and calling it a day. (Based on the chart, I could have gone with a 160 W, but I'm sort of into overkill)
