If possible fish are best treated in a hospital tank outside of the pond.
The best preventative of disease for healthy fish is understanding how to maintain good water quality and proper stocking levels of fish in regards to future size and water volume.
Most that salt their ponds have dedicated koi ponds, where it is basically a larger outdoor fish tank, where the koi are the focal point ie no plants, just fish in the pond. As was mentioned you are going to have problems if you have a typical garden pond/eco pond, as freshwater plants don’t tolerate much salt in the water. If these plants start to die it can throw off the balance of your pond.