It could be a close-to-full-grown Macrobrachium ohione (probably female, in that case), which stay smaller and are less dangerous to fish. On the other hand, it could also be a young M. acanthurus or M. carcinus, which are just as interesting and personable (will come to recognize their feeder) but do better in their own aquaria. The other possibility is M. olfersii, but you didn't mention asymmetrical claws, so I'm considering this less likely.
There's really no way to be sure without some photos, but I'll be glad to help ID them and provide care information.