I had a similar illness/lump on one of mine. None of the others were affected. It lived a couple of years with it and then it developed multiple similar lesions. Other than the appearance, the behaviour was unchanged. I assumed it was a cancer of some type. The feeder goldfish was about eight years old. It passed suddenly overnight. In nature it would have not received invasive medical exploration/ biopsy etc. I do not feel quilty in my neglect. I have spent thousands in treatment for my fur covered pets tumours.

I would assume tumors.

Thanks. That's what I figured, but I didn't want to leave it in the pond if it was something that would infect the other fish.
John