TetraTec makes a simple 5 watt UV
. It goes for about $90 online.
Other companies such as CustomSealife make more complicated (and probably better) UVs with higher wattage...
Many others are about 8 W.
I have the TetraTec 5 W on my 55 gallon. I can't say for sure if it is or isn't working, but I can say that I haven't had the algal blooms--that I was having problems with before I installed it--any more. Also, no bacterial blooms.