have a look at the search function on here, using UV as the search term; they are highly effective and I know of several people that use them to keep diseases at bay.
I myself use 4 in a commercial environment, to keep fish like neons and clown loaches from getting white spot - common in shops cause of stress and tanks being mucked around with a million times a day, and also in goldfish tanks to keep algae away and another on a display discus tank.
they work very well, and are worth the investment, if you really want one.