I have used UV sterilisers in my aquariums for a few years now, but I have yet to find one I am completely happy with. However I continue to use them as I do think that they reduce algae and can prevent some diseases. The only time I have ever had a white spot outbreak was when I wasn't using a UV steriliser. They don’t make algae already in the aquarium disappear, but they seem to reduce the amount of algae which forms and make it much more manageable.
The best one I have found so far is the All Ponds Solutions 9W UV steriliser, it is a powerful filter which also pumps air into the water and has plenty of filtration capacity inside, but the ON/OFF switch for the UV light doesn’t always work correctly (this has happened with 2 of these filters), so I have to completely unplug it if I need to turn off the UV light for any reason. I have also recently tried the Green Killing Machine, but it did nothing at all for algae, I think it is designed more for green water.
UV filters have to be switched off when using medications and some water treatments, and the bulbs need replacing around every 6 months.
