The tanks that I want duckweed, and in my case water lettuce as well, have to have an anti agitation device in place. With small tanks or non showcase tanks a piece of ridged pink or white Styrofoam blocking off a section works well. But in a show tank home-made feeder rings works fantastic for keeping your floaters both alive and corralled in the ares you want shaded provided your fish don't eat it all. Just hoop a piece of soft hose and join the ends with a double barb or a very tightly fitting hose inside or outside so it is air tight. Tie that in place where ever you want it in your tank and toss in duckweed and it will quickly fill your ring.