Donnie, tilapia will eat both duckweed AND watermeal, however with predators in the water, tilapia will avoid it for fear of getting eaten themselves. The best approach I have seen is to treat the duckweed (and watermeal which I see none of here) chemically. The tilapia, if stocked in the right numbers will consume most, if not all of the plant seeds that could reinfest the pond the next season.
Also, with predators in the pond...there is no massive die off of tilapia....most get eaten before dying and those too large for fish to eat in one gulp will get scavenged on by turtles, catfish, and all the terrestrial predators if they float up.
