Duckweed is a thick, green weed that grows on tops of water. It appears similar to certain forms of algae but is much thicker and denser. Once duckweed becomes a problem in a body of water it can be difficult to eliminate. However, introducing a fish population that looks upon the weed as a food source is one effective way of cleaning up the problem.

Grass carp look upon duckweed as a food source and may eventually clean up the duckweed in a pond, but are not 100 percent reliable. They are not the quickest option for cleaning up duckweed because these fish enjoy other types of plant life more than duckweed, and typically eat the reserves of those other types of plant before turning to the duckweed. Larger grass carp do a better job of controlling duckweed than younger and smaller fish.

Koi fish look upon duckweed as one of their favorite food sources. These fish love duckweed so much that you actually reduce the amount you feed koi in a pond if an ample supply of duckweed is present. The only problem is that it is difficult to strike a balance between the koi population and the duckweed. Too many fish in the pond and they will eat all the duckweed and need additional food again.

Goldfish are also used to control the amount of duckweed in a body of water. Goldfish make a good option for someone looking to raise an affordable, easy-to-look-after and common fish for a pond. Goldfish are also quite attractive if used in a water feature where fish are visible and will contribute their good looks while also helping to keep the pond free of the thick green vegetation.

Some people raise tilapia in ponds that are rich with duckweed because a strong duckweed population provides adequate dietary nutrients for the fish. Tilapia are excellent fish for eating, and varieties that sustain themselves on duckweed should grow to be healthy and large. Duckweed is high enough in protein that if enough of it is present in a pond, the fish won't need their diets supplemented. Tilapia don't do well in cold weather, so this is an option best used in warm-weather ponds.