We have a goldfish pond and the fish lived peacefully in it for years. Then the 6 largest fish squared off for a death battle. No known cause, but it might have something to do with breeding. Each fish got into a corner during the day to defend itself, then at night, they would battle. Ripped fins, bruised, eyes out. Then back into the corner during the day. This battle went on for about 4 days until only one fish was alive and it died from the wounds. I have asked lots of people about this, and never got an answer. Your question is the first about a similar incident ,i.e a death fight among goldfish. Totally bizarre behavior for a goldfish. The strange thing is that the smaller fish, a couple just slightly smaller, did not participate in the fight, and continued to live peacefully in the pond for years afterwards. Gold fish males will kill a female during breeding using similar tactics of chasing her and shredding her fins, and bumping her which will kill her. But it is not a fight, per se.