However if sissy is correct have you ever anethatised koi before Bob Z its quite a simple process and once under you'll be able to heck for stones etc.
Another thing that may well be happening is that your koi is shedding a tooth [they are constantly shedding throughout their lives].
So much so that we now have quite a collection of teeth eleven all told so far, both small and large.
If this is the case then after a few days it will have shed and should be back eating propperly again .
The teeth normally find their way into filters where if you are quick enough you can hook thm out.
The teeth on the larger koi look rather human in their appearance and are found at the back of the kois throat where they process food ready for it to pass through the koi's intestine taking out nutrients, protiens,vitamins etc before being passed out into the pond.
Dave