I can't get a puffer, can I?
I think it's a 20 gallon tall. I don't have any other fish in there right now. I was considering putting my betta in there but I also have a ten gallon so he doesn't need to be in there.
You could put a dwarf puffer in there - 2-3 if there's enough plants to really break up sight lines. There are also several Loach species that are rabid snail eaters, but they like tobe in groups and a 20T has a small footprint, so you'd be pretty limited there (they'd uproot stuff anyway).
If you go DPs though, you would actually have to find an occasional source of snails lest their beaks get overgrown. There are entire websites devoted to puffers - read up, then decide.