Duckweed is a green plant that grows on the top of pond water. Duckweed has the ability to quickly multiply and cover the entire pond. Not only is this unpleasant to the eyes, but it is very harmful to the fish. No sun is able to reach the bottom of the pond to keep underwater plant life alive and when this happens, the fish don't get the necessary oxygen they need to stay alive. Luckily, there is a way to kill duckweed if you see it getting out of hand.
Remove the duckweed with a swimming pool net. This is a good option if you only have a small patch of duckweed. You can simply scoop the duckweed out of the pond.
Use a herbicide that contains the active ingredient diquat. Diquat is very effective in killing duckweed in only a few short days by causing it to turn brown, wither and die. Mix 50 parts water with one part diquat. The amount you want to use depends on the size of your pond. Keep in mind that 1 gallon of this mixture will cover 1 acre.
Place the duckweed killer in a garden sprayer and spray the duckweed in your pond. Make sure the duckweed is completely soaked in the mixture.
Repeat these steps two weeks later. Your duckweed will be completely gone after the second application.