Over-sterilization, esp. in a reef tank is not a good idea. YOu will kill beneficial stuff as well as "bad" stuff. Find out from the namufacturer what flow rates you need for:
60,000 µw/cm2: this is the noraml "running" rate.
90,000 µw/cm2: to zap those nasty little parasites like Ick.
Normally, you want to match the unit so you get the 60,000 µw/cm2 exposure at a flowrate that turns your tank volume 5-6 times an hour... you'd want a unit that gives 60,000 µw/cm2 with a flowrate of 600 gph- probably a 25 watt.