SALT as a Medication:
Salt is probably one of the oldest forms of pond and water treatment available, although please always use a propriety pond salt. Not some bought from the corner shop for cooking as this type of salt nearly always contains the ‘extras’ that your Koi don’t require. Salt added to any pond or tank should be at the rate of 1/2 ounce per UK gallon or (12.5g) 4.55 litres. Never ever add a salt dosage to the pond in one go, do it over a period of several days so that your fish become acclimatised. We know that a lot of pond keepers still use salt as a preventative treatment, but there are others.
A salt bath is often the cure for many minor problems, like gill infections, raised scales caused by a bacterial infection, ulcers on the body etc. and a salt bath cannot do any harm to the Koi providing you don't go over the top with the salt levels, remember salt is an irritant in fresh water fish so encourages the shedding of their natural mucus, depending on the severity of the problem we suggest a salt bath with a level of between 1.5 ounces per gallon to 2.5 ounces per gallon .. but as salt can act as an anaesthetic after a period of time, then we suggest that you do not place the Koi into the bowl and walk away - otherwise you may return to find it is too late to resuscitate the fish.
BUT if you are using salt as a medication bath then you MUST ensure that there is plenty of air supply in the water - using at least a single 2" air stone - oxygen is extremely important as salt in the water, especially at the higher doses depletes the oxygen levels very rapidly.
Also, NEVER use salt in the water if you have any 'Formalin' present in that water, which you may have been using for other treatments, you may have inadvertently gotten away with this - but salt reacts with the Formalin and causes stress related problems to the Koi under treatment. If you are unsure about using salt baths don’t attempt it without guidance, or if you decide to run your system with a low salt level, use the rule of 1/2" ounce per gallon of water.