[DaveK]

My advice would be to save your money, and spend it on something that is going to do some good. No, a 25w UV unit would not come close to being able to control diseases. I am assumeing that the diseases you want to control are Ich (Cryptocaryon irritans) and velvet (Oodinium ocellatum).

There are people that use UV and strongly believe in it. After trying it several times, over the years, I have not seen any big advantage to using it. In a home reef, I think it has little to offer. It's a bit different for a store system. Properly installed and maintained, it can help to stop the spread of disease between tanks. However, this only happens if everything else is done right. This is something very rare at a lot of LFS's today.

If you want to find out more about UV, get a copy of the book Aquatic Systems Engineering by P.R.Escobal. The book goes into a lot more detail about UV, and how to size and use it.
For disease control, invest in a QT tank and always use it, for fish and inverts you want to add. Once you have a QT tank, maintaining high water quality, and not over crowding the livestock will just about prevent disease problems.

While we are on the subject of disease control, never ever medicate a reef tank. No matter what the label says, no matter what the store tells you, there are no "reef safe" medications. Remember, unlike medications for people, that much be proven to be safe and effective, there is no standard for reef medications. Anyone can make any claim, and most of the claims are questionable.