two of my fish died of unknown illness. I ran a uv filter for two weeks, then let the tank run for another two weeks with no fish. After that I introduced 2 domino damsels and after a month they and my live rock are doing good. My advise is run it for a short time at most and let your tank "mini-cycle" because you are killing a lot of small organisms that will cause a small ammonia spike. Let your tank be for a little while and then introduce fish one at at time. and see how they do! I have had good results with limited use of the UV filter, Hope this helps! Good luck!