The first thing you should do is to seperate the fish from the other healthy.
You should not subject your healthy fish to medication if only one fish has a problem.
It is much cheaper to treat only one fish in a hospital tank.
If this fish has an bacterial infection, it might infect the other healthy fish, or if it is a parasite, it might also infect the healthy fish.
I agree with Kohaku, you should always do a scaping when in doubt. Bacterial infections also cause the symptoms that you have mentioned.
You have now already treated with salt. When in doubt and you can not do a scraping, a salt treatment is always the best as an initial treatment. You could have increased the dosage to 0.5% (5 kg / 1000 liters) It will at least eradicate costia and most parasites.
However, you are precluding yourself from using certain medication like formalin because you have already treated with salt.
Do yourself a favor, get a hospital tank(at least 700 liters) and get yourself a microscope. (a small portapool will do) I believe that no koi-keeper should go without it. When you have done your diagnosis in your hospital tank, we can advise you on what medication to use.