Copper sulfate and Cutrine (9% Chelated Copper) are good algicides. They treat micro alge, and filamentous alge. I apologize if I'm mutilating the spelling of these words. Duck weed and pond weed can be controlled by a product called Sonar. Very pricy, but a little goes a long way. If you use Sonar or for that matter any of these products, they kill the alge / pond weed and the decomposing matter can cause an oxygen deficiency. This can kill your fish.
PM me if you need more info. I've experienced the good and the bad.
Mike
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