Most tumors are benign (non-cancerous) while, in rare cases, they can be cancerous. Some tumors are external. They occur on the surface. These can be surgically removed by a veterinarian.
Unfortunately, internal tumours cannot be operated. These tumors can grow and press against internal organs. The fish mostly die if they are affected by these tumors.
Tumors in the Gills can be caused by a thyroid malfunction. These can be cured by treating the goldfish with iodine.