[QUOTE="The downside is that unlike freshwater snails nerites are unable to right themselves if they are on their backs.[/QUOTE]
That's the urban legend, but I'm not sure it's true for all nerites. In particular, I think the 'Black Racer' nerites can right themselves, because I've never lost one in this way (but I do have sand substrates, which may help). The 'Black Racers' aren't as pretty as some of the others, but they are hardy, grow fast, and are awesome for algae control; they're the only ones I buy at present.