Pl*cos are infamous for being very healthy 'poopers'. I had one for a while and I ended up re-homing him.
I'd say guppies are a dirty fish also, mine always have poop hanging off them. They aren't the best eaters either, they nibble on a bit and let it fall.
Tetras like neons and diamonds are VERY clean eaters. I hardly ever see one of my tetras waste even a particle of food.