That's a limpet. Functionally and taxonomically it's a snail, but they don't multiply the way snails do and I couldn't consider them a pest the way some people seem to think of snails. They probably came in on plants, but they're in no danger of overrunning your tank. They will help a little with algae control, though, and they're really neat to watch. Their shells are quite efficient armor compared to snails, since they don't ever have to expose soft body parts. It's unlikely your fish ate them, but they do range all over the tank walls, gravel and plants, so at times you won't see them.