First off what is Kosher salt , I've heard of medicated salt and cooking salt both of which can be used to treat fungus, however looking at your koi this fungus is not recent and its not in our view as a result of chlorine .
First of all you need to be telling us all your water perameters because this type of problem is not a problem in a well maiintained system, the fungus will only attack reas of dmaged tissue so the arc of fungus , it is only present on your ki because of a break in the protective mucus coating on your koi , which most probaly came about due to netting damage although it may also be present , because of spawning overcrowding, parasite or bacterial infection , poor water quality or poor diet , infact any stressful factor so the key or sectret to you preventing it in the future is to provide your koi a happy stess free enviroment to life in .
To treat your koi please check the above to see what has allowed for this outbreak and correct it otherwise your fighting a looosing battle against it,
Do you know how to sedate your fish :-
https://www.gardenpondforum.com/threads/anethatizing-you-koi-goldfish-the-various-stages-of.10493/
Because it is best to sedate your koi and remove the fungus with a cotton wool bud, oce you have removed it then dab malachite green over the area of infection and if you have propolis spray that with propolis spray, if you dont have any you can buy it in spray and liquid form fromany health food store .
Once you have done this you then need to turn your attention to the pond and after correcting any bad practices treat the pond with an anti-fungal treaatment to elliminate any fungal spores
Dave