Found this online:
There are numerous reasons for tumors in fish and tumors are by themselves not to be considered as diseases or even negative even though they can have negative esthetical effects on the fish that gets them. This does however not mean that a tumor can't be a problem; they can be signs of very dangerous diseases. It just means that tumors aren't necessary a problem or a sign thereof. Tumours can for instance be caused by the thing most of us think about when we hear the word tumour – cancer. Cancer aren't common in most tropical aquarium fish as they are relatively short lived and cancer only rarely have time to develop and become a problem before the fish dies of age. This does however not mean that it never happens. Cancer is more likely and can be a bigger problem in more long living fish species. Sharks don't get cancer and you can therefore rule out cancer if one of your (true) sharks develops a tumor. Some researchers actually hope that sharks will reveal the secret to preventing and curing cancer in humans as well.
