Its true. They need to be in a tank with fast moving, top dwelling fish. But even that is no gurantee. They will eat feederfish without a problem. So it would be a good idea to feed sinking pellets so that they are less apt to "hunt" and more likely to scavenge. And if you do keep fish in the tank with it, don't get terribly attached to them.
By the way. If your buying it from where I think your getting it (petco) make sure that you inspect it for leaches.