I just noticed today that my goldfish has a lump on his head that is white around the edge with a red centre. He is eating and behaving normally. He is nine years old and has never had any other problems. The water quality is all came out fine when tested. Does anyone know what this is?
I think your goldfish has a virus infection. This might lead to a tumor forming later.
The only thing you can do about this is to be sure the water quality is as good as possible and to elevate the temperature in your tank to 22 - 24°C and hope your fish can fight this on his own (goldfish are cold water fish so a temperature of 15 - 25°C is ideal for keeping them).
Do not add any type of medication to your water, it will kill the good bacteria and is unlikely to help your fish.
Here is a list of diseases that carp fish can get 17 Goldfish Diseases. It is a scary reading but it might help you and others to diagnose and treat the fish in time.
Update on the lump:
I consulted some of the notes and websites that one of my lecturers gave me (I am a veterinary medicine student) and decided to treat it with a fungal treatment. It improved almost immediately and now, 4 days later, the lump is almost completely gone! Looks like Billy had a lucky escape!!