Was the goldfish wearing steel plate armor? My gars bite down on the goldfish so hard sometimes that the goldfish eyes pop out of their heads and nothing has ever happened to their jaws. If its jaw broke from just biting the fish it sounds like there was something already wrong with the gars jaw before that.
Chompers, the goldfish did nothing it was just lying in it's mouth, but it was too big, but I've always been amazed at what the could chug down so I didn't worry about this.