What Duane is referring to is known as black spot disease, and it actually can cause stress and possibly infection in an otherwise healthy aquarium fish.
The disease is spread by birds, and carried by snails. In freshwater fish there is no real treatment, you have to simply wait it out & make sure that the cycle cannot repeat itself. (do not keep snails with infected fish)
The "Noga" mentioned in the excerpt below would be Dr. Edward J. Noga, MS, DVM, a highly respected professor of aquatic medicine and immunology, that has been published approx 150 times in related papers/journals. His lab at NC State University specializes in the study of infectious diseases of finfish and shellfish.
According to Noga, if the fish has "black spot disease" there is no OTC treatment for the metacercaria (which are not in themselves lethal). They will simply "come out" of the cysts.
If there are no snails or copepods around, that is the end of the line. However, while the metacercaria are there, they can cause an immune reaction, which can be hard on the fish. If/when the metacercaria come out, they leave behind a hole that can get infected with bacteria and/or bleed. That is a danger to the fish. Salt in the water helps healing and increases production of the slime coat. Frequent water changes dilute out any possible infectious bacteria and supports the immune system. The key for fish survival (if they do not bleed to death from the holes) is a good immune system.
Black Spot is due to the metacercarian stage of a parasite with a complex life cycle called a "digenean" trematode . The "black spot" is caused by the metacercaria encysted under the skin which irritates the melanocytes of the fish tissue, causing the dark spot.
As long as it is under the skin, it is impervious from outside.
Personally I would pass.
