They will "nuke" any good or bad bacteria or algae in the water... benneficial if you add fish without using proper QT meathods (like everyone else) or if you have algae blooms, but there are other ways to take care of those problems. They are good in general if you have the money to spend on them. Useless if you don't change the bulb every 9 months to a year and keep the quartz sleeve clean, should be run continuously to get the full bennefit out of it. As I said they can be a great addition to a reef tank, but if you have limited funds you might find something better to spend it on instead, GFO reactor, quarantine tank, skimmer, RODI unit, efficient water movement. Personally I think all those should come before a UV in a home tank, but I have been called crazy more than a few times. (and by more people than my wife too:no