Yes, if you get one big enough. The whole idea is that the water has to be exposed to the light for a long enough duration for the nasties to die. So... that usually means that you get one large enough with slow enough flow for the water to pass through and kill the diseases. I have a 36 watt on my 125g tank. It is rated for a pond of 500g for controlling green water. Algae is easier to kill than bacteria and such.
Remember it help the already infected fish just the water born germs will be killed.
I got ick a few times when i was first setting up my tank and put a uv on there and haven't had anything since. I have a 9 watt on my 55 gallon i only run it about an hour each night when the lights go off and it seems to do the job..