In my view, there are 3 aquatic environments - freshwater, brackish and marine.
Koi and goldfish are freshwater fish, so they should be kept in a freshwater only environment.
Salt should only be used as as medicinal treatment in a separate container.
Some people like to use salt as a supposed preventative measure, but I see it as a measure that hides other shortcomings in the maintenance/setup of a pond.
I always try to learn what the origins are of where the fish/plants/invertebrates originate from and try to recreate that environment.
Shortcuts can be useful, but you must understand both the negative and positive aspects of the shortcut first.
.