Grass carp are one of the largest members of the minnow family, commonly reaching weights in excess of 25 pounds. Native to the rivers of eastern China and the Soviet Union, grass carp have the unique ability to eat and, therefore, control a wide variety of submergent plants. This behavior has led to grass carp being introduced into waters all over the world for aquatic weed control purposes.
Although grass carp are related to both common carp and goldfish, they differ in appearance and feeding habits. Grass carp lack barbels and spinney dorsal and anal fin rays. In addition, grass carp feed strictly by grazing aquatic vegetation and do not share the bottom feeding habits typical of common carp and goldfish.
Here is some info I got from the NYS DEC
Grass carp can be used as a biological form of aquatic weed control. While fertile (able to reproduce) grass carp have been used in many countries, the majority of states in the U.S. (including New York State) prohibit its introduction due to concerns about the fish reproducing and possibly destroying valuable wetland communities. In 1983, creation of a sterile (called triploid) grass carp eliminated reproduction concerns. New York State now allows the use of triploid grass carp on a limited permit basis.
Like any other weed control method, the use of triploid grass carp is effective in some situations and not effective in others. Triploid grass carp are most effective when used in small (less than five acres) weed-choked waters where low numbers of these fish can be stocked. DEC monitors the use of triploid grass carp by a stocking permit program administered through the Bureau of Fisheries office.
It is prohibited to possess grass carp without a permit. Anglers who may by chance catch a grass carp should immediately return it to the water unharmed.
