There are three basic kinds of filtration for a fish tank: biological, mechanical and sometimes chemical--biological for the beneficial bacteria necessary in the Nitrogen Cyle, mechanical for removing particulate matter, and chemical filtration sometimes used for removing chemicals such as medications used in treating fish. UV sterilizers are used for clarification and sterilization. They work well in treating green water caused by algae. Wattage, turnover rate, dwell time, amount of particulates are some of the factors that influence the effectiveness of UV sterilizers. Clarification is relatively easy to achieve, while sterilization requires more wattage and/or longer dwell times. Most fish keepers aim for clarification. With floor to ceiling picture windows and sky lights in our living room, our fish tank is in sunlight most of the day. We developed an ugly green water problem which the UV sterilizer quickly resolved. My fishes cuts and scrapes and other serious injuries heal very quickly, and I have long ago quit worrying when they get injured. I really think our UV sterilizer helps in reducing pathogens that could cause infections in our fish. I wouldn't want to run my fish tank without using a UV sterilizer, because for me, it's peace of mind.
Here is a definitive article on UV sterilization:
http://www.americanaquariumproducts.com/AquariumUVSterilization.html
When we first set up our 300 gallon tank, the water quickly turned really green.
The UV sterilizer quickly cleared up the green water