I've had some experience with GC. Years ago I have bad Eurasian Milfoil and put in 18 in my 1-acre pond and they cleaned the pond of almost every plant. So with my fish kill pond of three years ago I am starting over. Last year I had Curley Leaf pondweed witch will die back some as it gets hot but I put in 6 GC to help. This year my Curley Leaf Pond weed is worse. I am waiting until fall to see what it looks like and will probably put another 6 GC in and watch it for another year. Since I know what 18 will do to my pond I don't want that many. These fish are very smart and getting rid of them is hard. They used to come up to my deck when I fed them. I was going to get rid of a few and I tried to spear them with a large prong on a 10 long shaft and once I tried that they never came back to feed. They were so big, probably in the 20 to 30 lb range that the first one I speared pulled the spear right out of my hand and I retrieved the spear in shallow water. I did get one more chance and speared another one and the same thing happened. Thankfully the winterkill got all of them. So I think the secrete of using grass carp is to put in a few and watch them for the season and then a few more the next year if needed. Dolly eating the bones of one of the winter kill grass carp.