Let me tell you how bad duckweed can be. I had an over fertile pond from cattle runoff. First the pondweed and algae grew pretty thick. The the duckweed covered 90 - 95% of the pond. That shaded the pond like really cloudy days in Auguast. My oxygen levels dropped (I don't have a meter, just watched how my fish stopped eating and hung near the top). I would then go out and fill my john boat with it and my fish would start eating again. I had to do it every week or I had problems.
I'm planning to drain, clean and divert water and start over with well water.
I HATE duckeeed.
