You stated that you were running water tests in your post. Can you share them with us. Koi health usually starts with the water numbers, as those are the cheapest and easiest to fix, and then to parasites and then to antibiotic treatments which should be a last resort. If the water is not great, then all other treatments will not be successful, and if there is a parasite issue, fixing one spot does not fix the rest of the fish or the other fish.
Zone 7 A/B
Keep your words sweet. You never know when you may have to eat them.
Richard