Greetings I have a 18 Watt on my 210 gal. It is rated for 250 gallons. Am currently using a 404. Was thinking of getting another one and running them with 304's in my sump. A bit over kill. Instruction say flow rate is 200 - 400 gph but the chart on the back page of the manual says 110 gph for parasites. So depending on what you are using it for, check the flow rate chart and the flow rate of the pump. By the way, it cleared up the water considerably.
I'll give you a link on UV sterilizers, the long and the short is they are good for your tank, but not essential, but if you can afford one, you should use one. Also the author feels the Coralife brand produces a good UV sterilizer but it is over priced. He feels the same way about Eheims, good but over priced.
I originally purchased the 9 watt Coralife for my 75 gallon tank, but I just set up a 20 gallon, my 75 is still on the floor waiting to go, and installed it in this tank instead. When it comes time to set up my 75 gallon, I might use a brand of UV sterilizer J&L Aquatics carries, it has its on internal pump. (Sorry can't remember the name, but if this store is accessible to you, I recommend you check it out).
If you want really crystal clear water, use the UV sterilizer and add Seachem's Purigin to your canister filter - $14 at J&L Aquatics will cover up to a 250 gallon tank and its re-usable for 5 to 8 times, each time its suppose to be good for roughly 6 months - so cheap and works well.