I had the same problem in my pond in central TN. Tried a bunch of various chemicals at various concentrations with little effect. Maybe I finally got enough horse/donkey pee in it to kill the crap, but I'd like to think the swimming pool chlorine tabs I threw in helped! My pond is about 50'x70' and only a few feet deep. It is the only drinking water source for the stock, so I needed to be very careful with concentrations. This was hard to do considering I was guessing! I used either 3 or 4 of the big 3" chlorine tabs from Home Depot... just chunked them in, distributed around the edges, but far enough in to not be right where the stock drink. I repeated the application about every 2-3 weeks over the last half of last summer. My guess is that the tabs sink into the muck at the bottom and release fairly slowly.