Duckweed grows in ponds with little aeration or water movement and in ponds with excessive nutrient buildup. You can manage duckweed in multiple ways. Take a reactive approach and remove the duckweed after it grows, take a naturalist way or choose chemicals to control the outbreak. All are effective ways of killing or controlling duckweed in your ponds.
Skim the top of the water surface with a swimming pool net to remove the duckweed from the surface of the water. This is a reactive approach since you allow the duckweed to grow. If you have a healthy plant and wildlife ecosystem, this might be your best method for removing duckweed while preventing the deaths of other plants and aquatics in your pond.
Stock your pond with koi, goldfish or grass carp. According to Cornell University Cooperative Extension, grass carp will eat the majority of your duckweed but also will create a nutrient cycle that could cause an algae bloom. This is a natural way to kill duckweed in your pond; check regulations in your area to ensure you are allowed to stock your pond with grass carp.
Treat the water with herbicides that kill carotene. Some herbicides kill without the use of harsh chemicals that would hurt your aquatics, but the effect they have on Duckweed is the same effect they will have on many other plants. The use of these herbicides is considered less aggressive than most hard chemicals that could kill your fish.