I run a twin 30watt U.V on my 260 gallon catfish tank. they help to control protazoa such as white spot parasites. they will noy harm the filter bacteria because they only kill things that are in the water that flow over the bulb. they also kill things like plankton and algae keeping the water crystal clear. they work by "disrupting" the creatures DNA that pass over the bulb. the organism can then no longer function and so dies and is taken out by the filter system. I would recamend them for marine fish only tanks and freshwater tanks. they are expensive, but do have there advantages. I would say try and get a cheaper second hand one. (I think they are a little expencive for what they actually do.)