Don't let duckweed (Lemna minor), an aquatic plant that floats on the water, take over your pond. The plant is small--each frond on a plant is only 1/8 inches long--but multiplies rapidly. If not controlled, the weed will overwhelm your pond, depleting all of the water's oxygen and thereby killing your fish, according to Texas A&M University. Combine cultural and chemical controls to eliminate duckweed and preserve your pond's beauty.
Take away the duckweed's food sources. Scoop out any rotting vegetation or organic material, such as tree leaves, in your pond. Reduce the runoff into your pond or waterway, as runoff typically contains fertilizer and debris.
Remove the duckweed manually. Use a scoop net or rake and skim the surface of the water. Try to remove as much of the plant as possible, because branches that are left behind might grow into new plants. Lay the duckweed out in the sun to dry, then discard it or throw it in your compost bin.
Treat your pond with an aquatic herbicide. Use a contact herbicide such as diquat, which kills duckweed when sprayed directly on its foliage, or a chemical such as fluridone that's added to the water and kills the weed through absorption. Both are available at most pond supply stores. Apply the herbicides according to their labeled guidelines, since toxicity varies by product.
Repeat the herbicide treatment two to three weeks after the first application, if you are using a contact herbicide. This helps catch any duckweed that you may have missed the first time.