Welcome to the PB forum. Here are a couple of links with info on similar questions.

http://www.pondboss.com/ubb/ultimatebb.php?ubb=get_topic;f=6;t=000153;p=2

http://www.pondboss.com/ubb/ultimatebb.php?ubb=get_topic;f=6;t=000153;p=2

You are off to a good start. I would not add duckweed to a pond as there are better plants for ducks that are a lot less invasive and trouble than duckweed. You might look into poondweed (Potamogetons).