Lucky wasn't my fish. I was babysitting him, and he died earlier today. There were no signs. I'd done a water change that morning, but with nothing different, and after the change he acted normally. I can only assume it was age, or something pre-existing. I only had him for about a month, and wasn't that close to him--all he did was give me death-stares, even though I provided him with a bigger home and a healthy diet, but he was very entertaining to watch, and I think that he died happy. He'll be missed.