I have a gold fancy goldfish which has a white, lumpy growth on the side. It doesn't seem to be affecting the fish it still eats etc. All the other fish are OK, its getting bigger. Can anyone help??

Can you post a picture?

Sounds like a fungus infection.
Get some fish medication that treats fungus. I trust and use Jungle brand fungus fizz tabs.
It stains, so décor must be removed, but it's an excellent medication.

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Now that looks like a tumor. Unfortunately goldfish get them by no fault of the owner.
Depending on the rate of growth, your fish will live happily with the growth.
However their life span is much shorted than a fish w/o a tumor.
Not much treatment for tumors. You could try some melafix .

Thanks, I'll try the melafix