My parents had one that grew to 13 inches and lived to be almost 15 years old. I think that's a bit on the large size for a jack, as most sources say 8-10 inches. Really cool fish. He lived in a 90 gallon and killed anything we tried to introduce to the tank. He would just chill on the bottom like yours as well until feeding time. I know a lot are opposed to feeder fish, but we maintained a guppy farm to keep him fed.