A lot of times, issues like you speak of come on due to poor water quality. Water can be crystal clear and still be of poor quality. When was the last time you checked things like nitrates and phosphates. If you don't have any corals, you might not notice things like that. Corals are a good barometer for water quality but fish not so much.
I remember when I first got into the hobby, I had a fish die in my quarantine tank. I couldn't figure out why as there was no detectable ammonia, no algae, and the water looked great. Decided to check the nitrates and they were over 100. I can't recall what the phosphates were but I'm sure they were high too.
Not saying this could be your issue but I'd like to know your parameters and how often you check them. Do you have corals?
UV's work great and are a great addition to your tank. But if you aren't running a sump, what are you doing for nutrient reduction? Do you have a hang on back skimmer? It's almost impossible to keep water quality excellent without a skimmer or lots of water changes. How often and how much do you do water changes? How big is your system?
I know I'm randomly throwing questions out there but these are things that are important and anyone trying to help you will need to know.