Short term ( a few months) it won't hurt them. But not good as a regular diet because it is not formulated for them. Just keep an eye on your water quality because it may not be as digestible for the koi and can foul the filters faster.

We have a couple bags of cat kibble that I keep in the deep freezer and have been feeding my goldfish along with their regular food. They seem to be OK with it. They really like it. It has been 3 years now and no harm done.

Thank you both for the information. Yes, I have been and will continue to add it into their regular food. I feed them other things like peas, worms,crickets, watermelon etc. they are getting so big I can feed them quite a few different things now and they seem to love it. But those are all always supplemental items and not their main diet.

We noticed the labels on cheap pond pellets and cheap dry cat food look almost identical.

Not recommending, just sharing an observation.

yep....they probably are almost identical. And cheap pond pellets are just as bad for koi as cheap cat food is for a cat. Most (if not all) cheap pond food has corn as a major ingredient. Corn is not digestible and passes through as waste. It's basically a filler and ends up fouling the filters. Koi get nothing useful out of it. Poor quality food will shorten a koi's life due to fat build-up in the abdominal cavity and on organs.