I was checking out the pacific aquarium site, there is a link on the group order thread, and they have a pretty informative section called reef school. It had a section about uv so I'm just pasting it here for you. Hope it helps.
UV Sterilizers
Another method of water purification that became popular at that time and is still used today is UV light purification. The basic theory is that water is passed through a glass tube that is surrounded by UV lamps that destroy organic matter. It seems like a great idea in theory. Problem is that soon the glass tube becomes coated with debris and film and then the UV light can not penetrate to the water and thus never actually cleans the water. Also, the size UV system that would be needed would have to be massive to really have any real effect, not practical for most home aquariums. These units are often sold for fish parasite control, but again the same applies here, the tube becomes coated and not much is accomplished. And, typically not all of the parasites will be in the water column so even if the kill rate of the UV was 100%, which it is not in most units, the parasite may never be in the water column so it will never be exposed to UV anyway. My personal advise after testing and using UV units for many years is to save your money and don’t bother with one, it will do little more than lighten your wallet and give you a false sense of security unless you can afford a very large unit.