Duckweed isn't necessarily a problem in ponds. The tiny floating plants provide food for some pond animals and keep algae under control. Duckweed multiplies rapidly, however, and a small amount can extend to cover a pond completely within a few weeks. In excess, duckweed obscures your view of any fish and competes with other plants for nutrients and light. Duckweed can indirectly kill fish, because it blocks light to oxygenating submerged plants. All you need to keep the stuff under control in a small pond is a fine-meshed net and a spare 15 minutes.
Sweep the net through the top inch of water to catch duckweed. Stop when you remove the majority of the duckweed, or as much as you want.
Heap the duckweed at the side of the pond. This allows any trapped animals to return to the pond.
Shovel the duckweed after a few hours or the following day and place onto a compost heap.
Repeat every few weeks during the spring and summer.
Don't overfeed your fish and use the minimum amount of fertiliser for other plants, or don't fertilise at all. Excess nutrients in the pond encourage duckweed and algae growth.
Avoid using herbicides to control duckweed if possible. These can kill other aquatic plants and are toxic to fish.