Use of Rock salt in a pond can be helpful or disastrous.
When your koi or goldfish are sick Rock Salt could cure them or kill them!
DO NOT USE ROCK SALT IN YOUR POND IF YOU HAVE AQUATIC PLANTS OR GREENWATER ALGAE OR HAIR ALGAE! The salt will kill the algae and cause an oxygen depletion which could kill your fish!
Read this guide to know how to use this wonderful therapeutic properly.