Most people don't run UV on reef tanks as they kill good things in the water column. Some people do use them but you can have a successful Reef tank without.
depends on what application you want - some/most people who go fish only use UV sterilizers - UV's on full blowm reefs takes out some of the beneficial stuff for the success of the reef.
I used on for awhile, took it off and sold it 6 months later when my mandarin died cause the UV killed all my pods (primary food of the mandarin)
The UV will kill everything in the water column but does not remove them. You may prefer to opt for a Diatom Filter (such as the Vortex) to polish your water. You can run it on occassion to remove everything larger than 1 micron from your tank, and when you clean out the filter, all that "stuff" is removed from the system. only running it from time to time will allow micro-organisms to re-populate between uses.