How long has the goldfish had this, and did it grow gradually or just suddenly appear? How old is the fish?
If the fish isn't distressed, and is eating and behaving normally, then I wouldn't be overly concerned - they can live with these for many years. It is possible to have tumours removed by a fish vet, though, if you would prefer to have this done (but of course surgery is not without risks) - https://www.fishvetsociety.org.uk/inte ... illing-to-treat-pet-fish/
How are the water readings for ammonia, nitrite and nitrates? If any of these are sub-optimum, and chronically so, then they can lead on to health problems. Monitor these regularly and ensure that regular weekly water changes are done to maintain these at optimum levels, and that should help prevent future problems and at least give peace of mind for "managing" the tumour (as opposed to potentially worsening it / the fish's health generally).
Hi, thanks for the reply. This lump has grown gradually and it seems to have stopped but I can't be sure of that. This is a pond not a tank. I usually flush out some water every month with rain water from the water butt. I don't know about the water readings, I've never had a kit, but I've got a large biological filter that's been running constantly for two years, I've got absolutely masses of plants and the water is crystal clear. I feed very sparingly and there's never any waste. The other fish (goldfish, ghost carp and sticklebacks are all eating well and are disease free. Cheers, Si.