Most neoplasms (cancers) in fish are localised, benign to locally invasive. One issue is that the neoplasm can grow to such a size that makes them weigh fish down, interfering with their buoyancy. Consequently, for fish to maintain their position in the water, they will have to use excess energy. Of great concern in goldfish is that tumours can throw fish off balance, and lead to swimbladder disorders. Another factor for choosing to remove neoplasms is because they can grow but start to rot internally, causing fish to become ill. Some may be prone to traumatic injuries, causing pain and portals of entry for bacterial infection. For internal neoplasms, they occupy space and compress other organs causing them to fail.
Watch some videos of cancer removals in fishes at these links: