I had a rabbit snail in with a water plant, but it did some kind of algae bloom and the poor thing ... suffocated? Not sure exactly how that works. Expired, at any rate. But I have a large mystery snail in with the betta, and an ever growing assortment of tiny baby snails. They're a lot of fun to watch while brushing my teeth.

I do not have this many, though:
https://www.youtube.com/watch?v=l774T38hN_g