As they are in a tropical environment goldfish grow quickly. Are they all common?
The biggest one will be giving off a hormone to the other goldies that will stop them growing as big as he is. Goldfish will eat anything they can catch small enough to fit in the mouth. The only solution is to either take them to the pet shop or get them their own tank as commons can grow well past 12 inches.
Commons need 15 gallons each and fancy types around ten gallons each and 10 times the filtration too.

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