One grass carp will easily eat 4 to 6 times or more that amount of American pondweed. Age of GC makes a difference as to how much it eats per day or year. When those plants encircle the pond consider adding 1 grass carp to thin it out but not eliminate the pondweed. This is a very good habitat plant for the small fish. The plant growing along the edges also competes with filamentous algae and helps reduce amount of FA along the shoreline.