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
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
Love Tropical Fish? Find out how to create a beautiful, low-cost
tropical fish aquarium with complimentary tips at