They require very little. I have three in a 10 gallon and they do great. I have heard of people successfully keeping them in smaller. I would still recommend a 20 long though, the more room the better as always. They supposedly breed like rabbits, though I haven't had the pleasure of that happening yet. They live in colonies and do well together so you don't have to worry too much about aggression.
As for other requirements, obviously a cycled tank showing no ammonia or nitrites with some nitrates and LOTS OF SHELLS. I am working on getting a bunch of escargot shells to pile up in there. Don't bother decorating the tank with plants or driftwood as they will quickly bury it.
Some will say not to bother using any substrate, but definitely give them plenty of sand. It's so much fun watching them move it even if it does make a bit of a mess