[It is used on Fish Only systems. It kills bad bacteria. The problem is that it kills good bacteria as well. I read somewhere that there is one more problem -- if you turn off the UV then you fish seem less resistant to infection since they are used to such pristine conditions. That is what I read and it sounded reasonable to me, but I don't know if that is 100% true.
Basically, you don't need it on a reef tank because you want good bacteria.]
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