I am still a very new pond owner and was ever batteling the effect of green water and algae. I had drained and scrubbed clean my pond 3 times before I realized I had inadequate filtration. Forget UV sterlizers and algae killing chemicals. Chemicals just killed my fish and I have not noticed any difference in using a UV sterlizer vs not using one. All I can advise from my many trial and errors is OVER FILTRATION. I have 4 filters hooked up to my pond now, filtering about 7,000 gallons per hour (my pond is about 5,000 gallons) so every hour I am filtering more than my entire pond. I am in Dallas, Texas and with the constant sunlight and 90 to 100+ heat we have here, I was growing algae out of control. Now that I have an overkill of filtration, my pond has been crystal clear for 4 weeks now and I have not lost one fish. So anyone building their pond you can never go wrong with over filtering your water. Hope this proves useful.