They don't actually eat poop. Poop causes algae to grow rapidly, and the Tilapia eat algae - and other vegetable matter.
I know ducks eat duckweed - my friend's two runner ducks completely cleaned her pond of duckweed in a couple of weeks, and there was a thick layer of it all over the pond when they started. But I haven't seen geese eat it, and I know my breeder regularly has to scoop it out of his goose pond to keep it down. So I'm guessing duckweed would be a better choice than lily pads to keep down evaporation in a goose pond.
