Actually I consider a UV
a must have for any reef tank. I consider my UV
a main cog in the system for my entire tank. Not only do these cut down on the nuiscance algae the you get in reef tanks from all the lighting, using slower flow rates you can cut down on parasites and bacterial problems as well. Since much of what corals suffer from can be bacterial having a UV
can be a huge benefit to any reef tank.
As far as size, I have a 50 gallon reef with an 8 watt on it. Going with the 18 watt should not kill anything in the tank other than parasites and unwanted algae spores, although if I were you I would go with a 12 or 14 watt if you can find it. Aqua Ultraviolet makes some really nice set-ups - one with a built in wiper for cleaning, you may wanna check that out.