SO i read a post about what a uv light should do. Well i thought since i run a pond business, I should shed some light on the subject. The uv light only kills off single cell algae which causes green water. I does not kill or prevent the string algae or the stuff that collects onto your walls. People who use it in aquariums really only to sterilize their water. People who use it in ponds really use it to kill green water and have clear water. You have to get the right size light (watt) for the right size pond/aquarium for it to work properly. You must also not run the water through the light to fast.