Copper Sulfate. You can get it at any farmers co/op. A bioligist reccomended that I use it as some of the other products kill the weeds but the decaying plant matter kills the fish as well. This stuff worked for a summer but the grass moss came back the next year. I then put in some type of grass carp I again bought at the co/op and they flat took care of business and they cost me half as much as the copper sulfate.