Uv light usually for reef tank we don't use it. Hear say hear say it also kill beneficial bacterials. I have never use one before so can't really comment much.
I'm usually leave the fish alone as long as they are eating. Try to feed your tang seaweed more as it will naturally help boost up their immune system. I consider myself lucky to have my clown fish chromis wrasse and convict all snatching on seaweed haha which keeps them healthy
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