If we add low levels of salt to the water, then the fish doesn't have to work so hard to maintain the necessary concentration of salt in its internal body fluids. Salt also helps restore the mucous layer when it has been damaged by an exposure to certain potentially toxic chemicals, such as nitrite. Nitrite toxicity occurs when there is a build-up of this chemical from poor water quality. A salt bath of a brine solution that is approximately 2.5 lbs for every 100 gallons, for as long as the nitrite level is elevated, frequently for as long as many days, until the fish overcome the effects of nitrite toxicity. Other secondary conditions that are commonly associated with skin problems may be helped, but not usually cured, by the use of salt.
Ulcers are a common plague of koi and goldfish.
Unless the ulcer is very mild, salt alone will not cure it. Nevertheless, a salt bath will probably restore some slime coat and help an affected fish hold on until more powerful treatments are implemented. Drugs to combat parasitic and bacterial infections are also available to the hobby pond keeper, and several are just as effective as salt. Be careful not to use both salt and another treatment simultaneously. Serious harm can result to your fish.