I have a relatively small pond (about 250 gallons) with a large waterfall (about 10 ft wide and about 5 ft high). I have had this pond and waterfall for 18 years and, until about a year and a half ago, had no problem with algae using only a 3 inch chlorine tablet always in the water. But, about a year and a half ago, I suddenly developed a big algae problem. My landscaper suggested that I remove the river rocks at the bottom of the pond so that debris and algae could not "hide" under these stones. He also suggested that I add an algaecide and a phosphate chelator. I did all three but to no avail. The algae keeps rapidly returning (1-2 months). I drain the pond, partially refill spraying the sides with a hose, and repeat until I have crystal clear water. And, then, of course, continue to add the chlorine tablet, algaecide and phosphate chelator. But, the algae always continues to come back within a month or two. I am a retired Professor of Microbiology and my background suggests to me that I have selected a strain of algae that is resistant to all these control methods and, thus, the algae comes back to noticeable levels in a month or two (depending on the temps). I live in the desert in Arizona where we get a lot of blowing dust which settles in the pond which I suspect helps the algae. But, as I said, in 18 years, I never had this severe problem before . A friend suggested that I "shock" it. What is "shock"? And, what will "shock" do to the wildlife that drink out of the pools of water on the waterfall. We love watching all the wildlife drink from these pools of water which include rabbits (as many as 3-4 at a time), huge numbers of quail, huge numbers of doves, many, many other species of birds and also ground squirrels, chipmunks, etc. I do not want to do anything to the water that will kill these wildlife. CAN ANYONE OFFER A SUGGESTION ABOUT HOW TO CONTROL THIS ALGAE PROBLEM WHICH HAS OCCURRED ONLY AFTER 17 YEARS WITH THIS POND WITHOUT HARMING THE WILDLIFE WHICH DRINK FROM THE WATERFALL OR THE LARGE AMOUNT OF ORNAMENTAL PLANTS SURROUNDING THE POND WHEN I DRAIN IT ABOUND THEM? Thank you, Bruce