I agree with all of the above. We had a UV filter and it was great but now that it's dead the pond seems to be doing fine anyway. Summer of course is the hardest season for algae but if you put a good amount of plants in it will balance out. We also have a biological filter that we clean like once a year. I just spent two days pulling the overgrowth of plants out of my pond and the water underneath is crystal clear, and we have some huge koi in a small pond. The biological filter is just a half wine barrel (with liner) filled with lava rock that we pump the water through the bottom and exits out the top back into the pond. Then you add natural micro buggies that will live in the lava rock and eat algae and clean the water.