Quarantine

From Koipedia

It is always a good idea to quarantine new Koi before adding them to your pond. There are many koi diseases and many of them will transmit quickly and cause all your other fish to become sick as well. One of the worst koi diseases is KHV or Koi Herpes Virus which usually has no symptoms when the fish are below ° so you won't even know there is a problem until your pond warms up, and most of your fish quickly die. Survivors of KHV are rare, but when they do live they will still contain the KHV virus and should be humanly disposed of.