Goldfish Biting Each Other
(NPR, Florida )
I have several adult goldfish together. They’ve always all gotten along fine. One fancy goldfish grow humps on his back and another common goldfish which was smaller keeps biting at them and has eaten most of them off.
I removed the damaged goldfish. He is still alive and I’m wondering if the other fish was doing him a favor or if he was carnivorous.
Goldfish are omnivorous, so will eat flesh, but they generally aren’t cannibalistic towards each other unless there is a large size difference, or the fish are underfed.
I thought this may have been breeding behavior but looking at the image of the injured fish this isn’t the case.
Goldfish will pick on a fish in a group that they perceive to be ill or weak, but your fish looks healthy apart from the injury.
The growths on your fish have obviously attracted the attention of the Common Goldfish.
The growths could be either Carp Pox or benign Neurofibromas, nerve sheath tumors that cause localized skin and fin lumps that can grow quite large.
The action you have taken of separating the injured fish from the rest is the right one as you need to treat the injury with a light salt bath of two teaspoons of aquarium salt per gallon (un-iodized cooking salt can also be used).
Watch for signs of fungus or reddening around the wound, and increase the salt to 3 teaspoons per gallon if they appear. Keep the water pristine, as poor water quality will weaken the fish’s immune system and lead to further complications.
The injury looks quite deep, so healing will take some time, weeks rather than days.
Once completely healed, the fish should be able to be returned back to the community tank.
Watch for signs of the Common Goldfish showing too much interest in the Fantail again, which is a very nice example by the way, and you may have to separate them permanently.