Hmmm perhaps ponds are different but I had a UV on the tank to help clear a whitespot outbreak then, when I had fish dying in the pond, I added it to the pond. The fish stopped dying (but that may have been coincidence) but I had a sudden outbreak of blanket weed that I had never seen in the pond before. It started to smother everything. I turned the UV off and the blanket weed died. :-?
This seems the opposite of what others are saying on using them on a tank :dunno: