Koi and goldfish may fall prey to several different parasitic infections, including Epistylis, Trichodina, Chilodonella, Dactylogyrus or Gyrodactylus. It is best to obtain a skin sample and examine it under the microscope to determine whether external parasites are the cause of these symptoms. External parasites that are attached to the fish are killed when they are exposed to the rapid increase in salt concentration. Because there is so much more salt in the water than there is in the parasite itself, the parasite dehydrates and its outer membrane collapses, much like a deflating balloon or an imploding building. The level of salt concentration is selected so that it will be toxic to the parasite, but not to the fish.
Salt is also effective in treating injuries or infections to the slime coat of our pond fish.
The slime coat is the clear, mucous, outer layer of skin on the fish. It is a vital protective outer coating that wards off infection. It acts as a liquid band-aid covering over minor scrapes and bruises so that they can heal more quickly. Fish can fall prey to a host of diseases, both parasitic and bacterial, that reduce the outer mucous layer. This, in turn, makes the fish much more susceptible to secondary infections and illnesses.