You can see I am passionate about NOT adding "things" to ponds. Let me explain ....

Salt in a garden or koi pond IF it does any good at all is a mild tonic. Its effect IF ANY can only happen over a long period of time. Salt is an aggressive chemical which in certain circumstances can be lethal as you will see in a moment. The real technical applications for adding salt to a koi pond involve things like modifying osmotic pressure effects, to change electrolyte concentrations to reduce so called stress etc ... don't worry you do not need to know.

This kind of justification for adding salt to ponds is beyond my capacity to understand. On that basis alone I reject the concept ... and of course I have never seen anyone throwing salt into my local lake which holds carp (after all koi are just sensitive carp) anyway.

IF there is a real need to use salt to treat pond parasites then rather do the treatment NOT in the pond itself but in a large bowl and treat the fish with a much stronger salt solution for a shorter period of time.

Let's assume you have put salt in your pond and the problem of parasites has not disappeared. Someone then tells you through ignorance and not bad intent to add potassium permanganate to the pond.

These two chemicals can then react together under certain circumstances to produce chlorine which is deadly even in small amounts.

Potassium permanganate by the way is a common pond chemical .... it is that pink stuff used by dentists in their mouth wash

Salt is also bad when used in conjunction with zeolite.
Zeolite is used by some pond keepers as a way of adsorbing ammonia. When the zeolite is packed with ammonia and you add salt to the pond water then the ammonia is released back into the water in dangerous levels.

Do not add chemicals to your pond unless you are absolutely sure you need to and you know what the consequences might be.

More on chemicals and what they do and don't do next time. The biofilter properly sized, installed and maintained prevents more than 90% of all pond problems.