The 6 main types of freshwater snails:
- Apple or mystery snails. Poop lots, grow quite large and are terrible algae eaters. Breed readily but lay their eggs above the water line, so you can easily prevent them overpopulating.
- Ramshorn snails. Voracious eaters, may damage plants, breed like rabbits. Pretty, though.
- Pond snails. They seem to stay pretty small, but breed lots. I don't have a use for them, but they can have some nice shells.
- Assassin snails. They eat other snails.
- Malaysian Trumpet Snails. They live in the substrate and stir it up. Handy in an NPT.
- Nerite snails. Excellent algae eaters that won't breed in fresh water.
You can try shrimp with the less nippy bettas. :) I keep shrimp successfully with my male, but my sorority girls hunted them nearly to extinction last year. This year they are calming down and the shrimp are beginning to repopulate. I have red cherry shrimp, as I prefer them to ghosts. :)