The only way to eradicate Duckweed is with one of these products that
skim off any floating debris.
Even so it will take some time to complete the job as the duckweed gets
caught in other plants but you would notice a difference within a very
short period. You would also have to clear out the captured duckweed
quite often. especially at first, after that once a day is OK.
Clearing the surface of the pond of all floating debris makes a huge
difference to the look of a pond.

http://www.pondpumpsdirect.com/Oase-...s/c-1-324-330/


But if you cook the stuff and find you like it then it will dissapera
over night. Sods Law
Try googling for Duckweed cooking