That's a Procambarus clarkii crayfish. There are actually two of them in the tank, one large one and one smaller one. They are too large for any of the fish to eat whole, so none of them appear to view them as food/prey items. Plus I keep my fish pretty well fed, so they have no reason to bother with trying to catch a crayfish. They've been in there for at least 6 months now without incident. They're actually one of the neater creatures that I have right now. I got them from a chinese grocery store .