Sorry, I should have clarified...countryboy is correct about what the correct amount of UV can do, but I would assume the reason UV filters exist is because you can externally expose the water to high intensity UV light rather than placing one on the tank and risk exposing the fish (or yourself).
I doubt getting, for example, a UVA/UVB light bulb like you see for reptiles is going to be very effective if placed over a tank.
Is the water green cloudy, like from an algae bloom? Or white cloudy like a bacteria bloom?