Oh my poor love
I would say it is a tumour
Quote:
Tumours are common in goldfish: as long as they do not bother the fish (interfere with movement, bleed nor cause discomfort), most people leave them alone. It is possible to surgically remove tumours, but it?s an expensive, specialist task: many people choose euthanasia if a lump starts to cause problems.
The choice you have is, do you want your fish to live upside down like this, many fancy goldfish do due to swim bladder problems but they don't tend to have a great big tumour attached
sadly unless you know a fish doctor it may be kinder to put your goldfish to sleep
if you decide on the latter we can help you do this gently, where he/she will just go to sleep and not wake up