Using a skimmer to control duckweed
The only organic method for killing or controlling duckweed is to physically remove it either with a net or using an electrical skimmer.
It is usual to want plant growth across the surface of a pond but Duckweed - Lemna minor keeps growing until it covers the entire pond and will starve the submerged water of light, stopping the activity of the oxygenating plants.
You cannot use any chemical controls on the duckweed as these are not selective and will harm any other plants in your pond.
Unfortunately every last piece must be removed or it will spread back so if you want to stay duckweed free you must wash every plant carefully from an infected pond and remain vigilant to remove a plantlet perhaps brought in on a birds feet or feathers from a nearby pond as soon as you see it. Duckweed can drop seed to the base of the pond in Autumn to reappear in Spring.
These are the 2 versions of electrical skimmer unit available. The Oase unit is attached to a spare outlet on an existing pump and can be made as powerful as your pump will allow - also good for 'sucking in' fallen leaves in Autumn so they do not fall to the bottom of the pond. It operates by sucking and creating a vortex and the surface growth collects into the mesh collecting basket. The Velda unit comes with its own pump attached so there is no choice of pump size but it is easier to fit. It will not have such a good 'pulling power' to bring duckweed or leaves to its collecting basket as the Oase unit.