I apologize that no one has responded to your question sooner. Different experts come online at various times. I just came online and saw your question. My name is ***** ***** I’m a biologist with a special interest in fish. I'm sorry to hear of this iproblem. If you still need help, some additional information will help me determine the best steps for you to take.
What are the symptoms your koi has - appetite loss, skin sores, white fuzz growing on him, swimming problems, gasping for air, etc.? The more detail you can give, the better.
Do you have other koi? If so, are they all right?
Have you tested the water? If you have, what numbers did you get in parts per million (ppm) for ammonia and nitrites?