I've had several goldfish over the years that have developed lumps and
bumps. One grew a bump on the top of his head, so I named him
"Knothead." All seem to be benign growths, and none of the fish that
ever had these got sick or died. In most cases, the bumps have
gradually disappeared over time. I still have Knothead, but his knot
isn't as prominent as it used to be. He's one of the elders in my
pond and is about 10 years old now.
Gary