Ken start with the link below

https://aquaplant.tamu.edu/management-options/common-duckweed/

There is a lot here on the subject and you have correctly noted the difficulty of dealing with it.

If you want to save the fish then a combo of methods is best IMO. A contact herbicide followed by a systemic herbicide. You can work in mechanical removal to help the process. Because of its speed and reproductive methods it is hard to overcome.

Last edited by ewest; 05/28/18 10:00 AM.