My brother in law had a Goldfish Tank (Orandas, Bubble Eyes, Black Moors) with 2 Commons, These things were WELL Fed. One day when my bro in law came home every goldfish in the tank was dead on the bottom of the tank with a ring on each ones side or belly. Don't know which common went on the rampage but something pissed someone off. He brought the commons to my place and dumped them in my Koi pond while I was at work thinking they'd be fine in their (Koi were 14"+) . I had to hunt the little buggers (bout 9" a piece) down when I got home and donated them to the LFS. Makes me wonder if the Gibby in my tank will ever have a go at one of my Oscars. He almost as long as my Biggest O.
Mike