Have you checked your levels recently? I love what a UV does for water clarity, but wish we could figure out the cause of the cloudy water. I'm just concerned the UV is going to mask an issue that needs taken care of. How long has this tank had fish in it now? What is your stock now? How did you cycle it? It sounds like you're on the right track with the water changes, thinning the stock, and the powerheads, since it's clearing up a bit. What color is the cloudiness? If you could get a picture that might give a better idea.