Some critters eat "leftover" food, but ideally you just feed enough for the fish you have. Bacteria eats poop, which ultimately turns into nitrate, which gets "eaten" by plants, but leaving the poop in there is not a good idea. You could get higher nitrate levels than your plants can handle and there will be a buildup of other substances we can't test for.
So, to make a long post short... Yeah, what granny said.
EDIT: If they told you snails eat poop, I'd say they're full of it. I haven't seen anybody saying snails eat poop. Algae, yes. Poop, no.