Dave, I looked on the internet and I see not too many people feed there koi dog or cat food, but I am not the only one, and I have heard of many people giving there koi dog biscuits with no I'll effects. My koi just love the puppy chow and prefer it over the real koi food. The koi food around here is so small and my fish their mouths are so big, it's mean to make them chase the little pellets around. Also the koi food is not that fresh compared to the dog food, this dog food that I give is "large breed grain free puppy chow", and yes I give Puppy chow to my 7 year old dogs because they like the puppy chow better too. My puppies hate "adult" dog food and my big koi hate "koi food! Yes, during the summer I indulge the koi's wishes, but that's only part of their diet. The koi also eat bugs, lots of fresh pond greens, and they eat the gooseberrys off the bush that hangs over the pond too.
My koi are just fine and I have been feeding them this food for quite a few years now and they look real good to me and grow very fast during the summer. I have never lost a koi, of course I have only eight koi. The two biggest koi are 22 years old now, and the others are four and five years old.