Common duckweed (Lemna minor) is a rapidly spreading aquatic plant that deprives ponds of oxygen and leads to the death of fish and beneficial algae in still waters. It is important to get rid of duckweed for the health of your pond and existing aquatic life. Having said that it is generally found in nutrient rich ponds.
There are NO chemicals that can get rid of duckweed, as any chemical would kill off any other plants.
Duckweed arrives from plants that have duckweed on them, also from birds, or any other creature, that have been into a pond with duckweed before entering yours.
A pond skimmer is ideal to have to hand in any pond. It floats on the surface of the water removing films of contaminants, toxins and surface waste. The problem I persoanlly have found, these are very good but duckweed likes to get tangled in the roots of floating plants and stems of lilies etc…
Get rid of duckweed before it takes over the pond by increasing aeration at the bottom, manually removing plants from the surface and reducing nutrients that create bottom sediment.
Add natural predators of duckweed, such as Koi, grass carp, goldfish and domesticated waterfowl. Alone these biological controls cannot remove the problem, but if introduced to the pond before duckweed begins to grow, they can assist with duckweed control.
Read more at Gardening Know How: Controlling Duckweed – How To Kill Duckweed https://www.gardeningknowhow.com/plant-problems/weeds/duckweed-control.htm
A plant that uses the same enzymes and helps to combat the spread of duckweek is
Veronica beccabunga (Brooklime)