tkjeffrey, looks like I opened a can of worms for you....I would agree with what toadfish says...just run it from time to time......maybe when you have new additions, or your fish appear to be acting funny, but not all the time or you will lose some of your tanks diversity.
It's always been my understanding that a UV sterilizer will do exactly what it is designed to do, that is kill off all the micro-organisms that may inhabit your tank. Realize that this means all of them, it does not have the ability to distinguish between the ones you may want and the ones you don't. Everything that I work to maintain from live rock and a dsb would be killed off if I used one. Phytoplankton also would serve no purpose once it ran through the sterilizer, it would then be simply dead phytoplankton. I have never seen one in use in a reef tank, and never wanted to gamble with mine.
Pirate, It all depends on the size of the sterelizer. If you use one that's rated for half your tank size then it may help you control populations or unwanted materials. You are right however, that everything that passes through gets zapped. But if it's rated for half the system then everything doesn't pass through.
Do a search about them. This is a long time large debate as to whether they are good or not. Unfortunately our little systems don't breed/culture enough plankton to make a difference, let alone enough to feed the system. Our tanks do culture/breed far more bacteria, both good and bad, then any reef ever will and that's where a UV sterilizer will do the most good.
Why not post this same question to the Think Tank?????