One of my goldfish has developed a large growth on its back?

it has also recently started to headbutt my other fish please help? should i seperate them?
Answers:
You haven't described the growth. Is it an open sore or a bump that appears to be under the skin or something that appears to be growing out of the skin or what. It may be a tumor. Most tumors are a result of a virus entering an open trauma to the skin. Those viruses are Papilloma, Carp Pox and Lymphocystis. Tumors can be removed by first introducing clove oil as a sedative and then cutting out the tumor, but as clove oil is also used to euthanize, using just a bit too much could result in the death of the fish, and I, myself, would be too hesitant to perform such surgery. Tumors are usually benign and not a direct threat to the fish itself and they aren't contagious, but they can invite secondary infections and can compromise the fish's ability to swim, thus causing stress and exertion, etc. as well as possibly affecting its ability to eat. If your fish does not appear to be affected by this, I would say don't worry about it. I would always keep an eye on it though for any slight sign of infection and immediately treat it. Although the headbutting could be related because stress or infection causing distress, it really shouldn't be related to the growth.

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I would certainly separate them if this fish is being agressive. You might try and clean the water, as the fish may be stressed. It could have white spot, if the growth is not large and there are other signs of small growths. If it is a large hard growth, then maybe a trip to the vets to see if it can be removed. Golfish can also get a fungal disease which looks like cotton wool, this can be treated with some water treatment remedies. Your local pet shop owner should be able to help with which treatment is best for your circumstances.
Fish do get stressed easily and the water condition should be checked too.