Although there is little proof, rare cases have indicated the possibility of cannibalism in goldfish. The National reported an incident in which goldfish ate other fish after an earthquake, leaving them of food for an extended period of time.
Experts cite lack of space as the primary cause for fighting between fish, and state that a minimum of 10 gallons of water is required per fish They recommend adjusting the set-up of the aquarium and using an aquarium divider in some instances. Goldfish are omnivores and enjoy brine shrimp, tubifex worms and mosquito larvae as well as algae and aquatic plants. They also enjoy eating softened vegetables. Experts caution that a little bit goes a long way, and it is important not to overfeed them.