But they may cause more poop than they clean:
"The Common Scat is omnivorous and an indiscriminate eater. Food includes vegetables, small animals and debris, and they usually readily take frozen and flake food. They also tend to eat aquarium plants. And due to their eating habit they are quite messy fish, requiring a strong filtration. In seawater, their messy eating habits make buildup of toxic protein decay products a significant issue to be controlled."
(From Scatophagus argus - Wikipedia, the free encyclopedia