Maybe you are thinking of the boxer shrimp, which could be mistaken for a crab because of its ornate claws. Boxer shrimp as they are often called are actually banded coral shrimp. You can keep them in a fish only system, but they should not be added if you plan on having corals in the future.
Also only add one per tank, unless it is a true mated pair, as they will battle to the death. They have quite a personality on them and you can often catch them taking swipes at whatever passes by. Be prepared to cringe as they reach out for one of your pricey fish...lol. HTH