Please give some more detail on the tank. How big is it? What fish do you have? How long has it been set up? What are your water parameters (pH, ammonia, nitrite and nitrate)?
What is your typical water change schedule? How much and how often?
What do you understand about the nitrogen cycle? Did you cycle the tank before getting your fish? If so, how? Or are you now cycling the tank with fish? Or is this the first you have heard about cycling a tank?
Can you give some detail on the UV you are using? Make/ model. Or a photo if nothing else.
Many things can cause fish deaths in new tanks. The most probable cause is an uncycled tank. Incorrect use of UV could be an issue, but if used correctly shouldn't be an issue.
