Well, there's an apple snail species that can get to the size of a baseball. Like all apple snails, it lays eggs above the waterline, so they'd have a better chance of multiplying than most inverts. The only problem is, like with any large snail, a death will quickly pollute the water. They eat, algae, but when it comes to your plants, they only prune off the dead parts.
Another option would be the Maylaysian snail, which stays tiny, but also stays out of danger. These guys are a helpful substrate-dweller that will come up at night. Sort of an aquatic equivalent of earthworms. They're hard to buy, though. Typically, if you have them, great (so long as you don't keep spiny eels, but that's another story...) and if you don't, you don't. Interesting fact: Because of their shell structue, they're the only snails that can cohabitate with loaches.