Hi JimG...
I am not going to pretend I'm an expert... This learning about koi is all new to me, to tell you the truth....
Exactly what are you hoping to attain by fasting your koi?.... Is it for the purpose of inducing a breeding method, or to cause the koi to spurt in growth at a latter cycle...???
Here is a thread from Koi-Bito from last year that might help.....
Koi Bitto #8- The 80cm Quest: Part II