Duckweed grows in ponds that are stagnant, contain fertilizer or manure runoff, or don't have enough agitation and aeration from wind, waves or running water. It can be quite a nuisance in decorative ponds and can take over the entire surface without some form of duckweed control. Options to kill it include herbicides, aeration and adding creatures that eat the duckweed.
Disturbing the water surface will make the pond a less friendly environment to duckweed. Many pond owners do this simply by raking up the duckweed as it arises with a long pool-cleaning rake. Another option is to install aerating devices at the perimeter of the pond, or adding fountains to the pond to keep the water surface moving.
Aquatic herbicides are available from pool, pond and landscape suppliers, and range depending on the season, severity and location of the duckweed. Many of them require multiple applications, and the pond will likely not be safe for use during treatment. Some pond herbicides used by pond care companies against duckweed in particular are PondWeed Defense and RedWing. These are best used in ponds with large water turnover or ones that are not contained. Another herbicide designed to work on duckweed is called Reward.
Some fish, when introduced into a pond, will clear it of duckweed. Goldfish, koi and carp will eat duckweed in some amount. Grass carp are commonly used for this purpose and work well in smaller ponds. However, putting these fish in a pond also will incline the pond toward algae blooms, as they create an organic cycle of waste and food in the pond. Note they are restricted in some areas, such as near the Great Lakes and their watersheds.
There are a few pond herbicides available that will kill off duckweed and other aquatic weeds by non-toxic treatments. These include White Cap and Sonar, both of which work by keeping the duckweed from producing the carotene it needs to survive. Without carotene, the chlorophyll in the weeds breaks down quickly and the plants will die. These products are often considered safer than harsher chemical herbicides, but work best in larger ponds.
If an overgrowth of duckweed persists, the soil in the pond bottom may be full of plant-supporting nutrients from farm runoff or animal waste. To fix the problem once and for all, consider having the pond bottom dredged up and removed.