Maybe I'm wrong here but just wash out the minnow trap with freshwater. The duckweed should float off the trap then realease the minnows in the pond. It should not be hard to keep the duckweed out just be aware of it. I have had clients put in plants from other sources and they never paid attention to the little green things floating along with the plants. Just being aware of it should help. Also if it is just starting to show why not try to rid of it with a Reward application?
