Duckweed is an indicator of highly nutrient rich waters, and minimal wave action.

Bacteria and aeration will help, but to turn around a pond that is like that is like using a 5 gallon bucket to empty a swimming pool that has 4 GPM of water entering it.

You can empty the swimming pool, but it will take a long time, and you have to keep at it or it will fill back up again.

Draining, removing the nutrient rich organic layer on the bottom, THEN aerating and using bacteria after it refills would be the route I recommend.

www.hoosierpondpros.com

[Linked Image from lh6.ggpht.com]
http://www.pondboss.com/subscribe.asp?c=4
3/4 to 1 1/4 ac pond LMB, SMB, PS, BG, RES, CC, YP, Bardello BG, (RBT & Blue Tilapia - seasonal).