Tropical fish are like other species in many respects, and that includes the fact that they have diseases or disorders that may resemble those of other animals, but are specific to the fish themselves. One good example is tumors.
Just as humans, cats or dogs may get cancer or other types of tumors, so can tropical fish. The main difference is that treatment in fish is nearly impossible, depending on the type and location of the mass. Not only are they difficult to handle where a human can have clear visual access to them, but such things as surgery are basically out of the question.
Tumors generally show themselves as a lump under or on the skin of fish. These are a distinct bump or lump, as opposed to the white fluff of skin diseases. For the most part, tumors are benign, although it's possible for one to grow so large that the fish's quality of life Declines and you may have to euthanize them.
One type of tumor that does have some success with treatment, is the kind that forms under the skin of the gill, causing it to remain open. The cause of this is usually a thyroid malfunction. Remove the fish to a hospital tank, and add 1 milligram of potassium iodine for every gallon of water. Improvement can be slow, and the full course of treatment can take up to four weeks.
Internal tumors can be quite advanced before the fish shows any signs, such as a swelling of the abdomen. These cases are invariably fatal, with the rapid growth causing the fish to lose its ability to swim or eat, at which time you should consider euthanasia.