Duckweed (Lemna sp.) is a very prolific and hardy floating plant that regularly appears in ponds or is introduced into aquariums along with other floating species.
In the case of ponds it is often introduced on the beaks of birds or other animals as they travel between ponds. In many cases Duckweed is considered a pest due to its tendency to rapidly spread and block light to other plants. If it is controlled by regular removal however, duckweed can do a great service to the pond environment by removing pollutants and toxic compounds at a greater rate than almost all other plants. It is so efficient at doing this that some fishkeepers deliberately set aside areas for duckweed to work as a 'vegetable filter', which often has a great effect in reducing algae in the main pond.