Use the polyquat. It worked for me. Got the tip from one of the threads here, and it worked. Thing is, this year its seems the little buggers won't go away with a shock level of cl, so I tried the polyquat algacide. It did cloud up the water once it was killed, but I used a floc to let it settle to the bottom. Vacummed it to waste, and presto, no more algae since beginning of August! Have had clear water since.