Hi-
I'm guessing that the Koi gurus are going to need a little more info.
1. What are the current water parameters?
2. How many koi? How big a pond? How big are the koi?
3. Any signs of rot or fin problems?
4. Pictures of the koi in question?
I am not qualified to make a diagnosis on koi, but I'm not bad with goldfish. The reddish color in goldfish can suggest ammonia toxicity, among other things, or septicemia. I'm pretty sure that if you post some of the information above, the real pond people will be able to help you, pronto!