Perform a fifty percent waterchange, and clean the pond as well as reasonably possible without causing undue delay in treatment. Add all at once in the case of epidemic mortality. A word of caution on SALT though. It should be food-grade or cattle feed grade, NON iodized salt. Brochis britskii cats, and native or wild caught Discus.
Rams dislike it also.
Plants perish in the salt dose as listed above. Do not use it if you have live plants. Note the divided dosing, so as not to add it all at once. We saw very good results, but some precautions apply. Avoid contact with treated water.