So sorry I had missed this before!!

Nerites are a perfect brackish snail. There are several available for aquariums, including zebra and virgin nerites. I have seen various ones come through for reef tanks before, and they can be acclimated to brackish tanks (I think we had talked before about the changing salinity and how brackish animals are used to it–same goes for the snails).
I know there was a great plant site that actually sold the Nerita virgineus (I think that's what they're called) as freshwater algae controllers, though they can tolerate and breed in brackish water.