I would not think a 5 watt would work on a 50 gallon tank. Read about flow rates as well. If you exceed the flow rate of the UV
you are rendering it useless, since the water is not in cotact with the UV
bulb long enough to "kill".
EDIT - I just read the specs on this unit. Says it's good for upto 100 gallons and has a max flow rate of 300 gph
I still don't know about this though... 5 watts just does not seem like alot of light. And it's classified as a UV
clarifier. Not a steralizer.