Elfdoctor is correct. While a UV could be valuable in FO tanks where the hobbyist just doesn't want to set up a QT, when you apply the UV to reef systems, or even LR systems, the UV will kill off indiscriminately both good and bad micro-organisms. And since this is the food source for sponges and a multitude of other marine reef and rock creatures, it is not something you want to do.
And, here is something to consider: For the price of a UV you can set up an awesome QT and get a lot more bang for your buck in effectiveness.