Why do goldfish attack another goldfish?

ID3541807423 says: They aren't attacking each other, they are just playingPuertoRicoRocks says: Actually, it is rare, but goldfish can fight.But when goldfish are kept together it benefits them. (Unless it's a regular goldfish with a fancy goldfish, because in that case the regular goldfish would eat the food up before the fancy one gets to it.)It benefits them because they're not alone and they will swim more often, have more fun, and live a better life