Lemnodieae (the ones shaped like duckweed) have aerenchymal sacks that are filled with air. Wolffia and wolfiella have a waxy coating, which causes buoyancy. The waxy coating "works," but not as well as the aerenchymal sacks. If you stir up a mixed culture, you can see the difference.I watched a video once about a freshwater fish farm that had oodles of duckweed. Apparently the mechanism it uses to float is different than other floating plants? Many floating plants have a sort of air "bladder" where as duck weed is buoyant due to some sort of oil it produces? The fish farm was looking into if the oil could be removed and used as a bio fuel. . .
This was just some rando youtube video so who knows. . . maybe @Randy Holmes-Farley could tell us???
Also, goldfish love eating duck weed so there's that