Best product for duckweed is a slow release Fluridone, name brand or not. You want to kill the emergent and the preemergent as well. Once you knock it down or out, then try methods to prevent it. Much different from eradication. 2,4-D and Diquat work as well, but a lot of times you need to retreat because of the new growth that was hidden from the first treatment. So factor that into your overall costs.
