pathogens are probably breeding quicker than you can suck all of them through the sterilizer at the required rate to kill them all

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:

It's likely you had algae in the water that was using up all the nutrients, killing the algae would stop it being consumed and the dieing algae would also add to it. Blanket weeds feeds by pulling it's nutrients directly from the water unlike most plants that have there roots in substrate