It is possible! Sometimes the bigger goldfish of the tank gets kind of bossy and starts to think he is the "boss of the tank" and harrass the poor little ones around. This might result in your otheer goldfish being a bit quiet, or hiding behind ornaments more. It is never good to have a bully goldfish. However, bully goldfish don't always happen.
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No they rip each other!
Not usually. They are very good at living with others. That's why in pet stores who see them all in a tank together.
Goldfish are not particularly aggressive but given that they are omnivourous they will certainly eat other occupants in their envirionment if they are small enough to be swallowed.