I've only heard of this from one other person I know, and I did think it was the strangest thing I have ever heard. I've had goldfish all my life that have lived for years and I have never had any of them eat each other. All goldfish I have known would never eat another goldfish no matter if it was 10 times smaller than it or not, goldfish are usually such friendly creatures. My goldfish won't even pick on a dead fish.

I don't really know what to say about that goldfish except that it obviously doesn't mind living alone. Do you know what breed of goldfish it is? That could possibly help to figure out why its so aggressive too. All I can really say is that since he eats everything and everyone you shouldn't put any other fish in there with him, but you've already said that lol.

But yeah, you should see what breed of goldfish it is.