If you are going to salt, get a salt pen to determine the initial salinity, and the salinity after the addition (using the salinity calculator at the top of the page) of what you think will give you a change in salinity of 0.3 (half what Russ has suggested) as many pond owners are told the volume of the pond, but when checked against salinity, which will measure the entire system, they may have as little as half of what they were told. This will allow you to hit the 0.6 that Russ mentioned, and allow you to make subsequent additions of salt to maintain the salt level following water changes. It will also give you the true volume so you won't overdose any other chemicals, killing the koi.
I would still like to see the test results of the water chemistry. Regardless of what treatment is administered, the fish has to have a good healthy environment to heal and survive. New systems are notoriously bad due to the lack of time needed to bring the filters through the cycle and impatience on the part of the owner.
Zone 7 A/B
Keep your words sweet. You never know when you may have to eat them.
Richard