If you have not introduced koi to your pond for a long time, you need to be extra careful. Simply putting our koi into your pond may cause some trouble to either the existing koi or the new koi. It is not because the new koi is sick or because the old koi is sick. It is simply due to different immunity levels.
Without having any new introduction of koi for a long time, your pond has established its own eco system. When new koi that have been raised under completely different surroundings is introduced to your pond, it affects the balance of your eco system. The new koi may not have immunity to what lives in your pond; whereas, your existing koi may have developed immunities over a period of time, and Vice Versa.
So, when you have completed the normal procedure of quarantine, then you can introduce one of your old stock to the quarantine system. Raising the salt level to 0.5% is a good idea at this time. After making sure nothing unusual has happened in the quarantine system, then you are ready to introduce your new koi to your main pond.
Read all the details of koi care after receiving in your home.
Read about how to setup koi quarantine tank