I once kept a six inch large mouth bass as a pet. I caught a huge (5") crayfish and put him in the tank to clean up. The bass grabbed the crayfish by the head. The crayfish clamped on to both sides of the bass' jaw with his pincers. They swam around together like that for about 3 days before they reached that "tense state of mutual tolerance".
Growing up we had a 6lb bass in a 55gallon, he was 1lb when my dad caught him. It was amazing to watch it eat the feeder fish. We gave him hot dogs as treats, he loved them.