although wamp is correct by saying they are VERY difficult to keep alive in a tank, you can keep a few in a tank. Although it is probably not a good idea, because they will most likely die within a year and when they die they can easily wipe out ur tank if its not very large. But, if you want to try them out, then give it a shot, you can have more than one. Remember that they move a lot and you don't want them on top of each other!