Grass carp are herbivorous fish that feed on duckweed, a flowering plant that coats the surface of some ponds and lakes. The fish, which is native to Asia, can be used to control the duckweed. To keep the grass carp from entering the ecosystem chain, however, it must be constrained and not allowed to migrate beyond the targeted body of water. The fish are intended to reduce the duckweed---not eliminate it completely---and can only be used with a permit.
Contact the department of fish and wildlife for your state, which will be able to tell if you are permitted to use grass carp to control duckweed and provide names of vendors from which you may buy the fish if and when a permit is issued.
Check the body of water for any inlets or outlets along the edges. These must be screened to keep the carp from escaping. Before placing any such structures, you must obtain permission from your state.
Buy your fish. The number you purchase will depend on the size of the fish and the size of the body of water and the amount of plant matter available. Generally, nine to 25 grass carp of 8 to 11 inches in length will keep an acre of water under good management. Controlling duckweed with the grass carp will take a few years, especially if you start with smaller fish. The carp will grow up to 18.1 Kilogram and need enough space to live without being crowded out by other carp, so it is better to have too few fish rather than too many.
Introduce the fish to the water once all the requirements of the state have been met. Think of this as a long-term project and one through which you are trying to achieve a balance between nature and use of the water. When the grass carp are feeding heavily on the duckweed---during the summer months when it grows rapidly---there will still be small areas where the weed will continue to grow, unless you have overstocked the waters.