I'll stick my neck out to have it chopped off by the pundits - but the BEST snack and food for Koi is freeze dried Krill - HOWEVER it is not the Koi's diet, or staple. They need "real food" too.

Koi eat a lot, Goldfish do too, but Goldfish are better foragers. 

If you took two identical ponds and you neglected to feed both ponds equally, the population in the Koi pond would die out faster than the population in the Goldfish pond. Part of the reason is that Koi eat more, so they starve faster. Part of the reason is that Goldfish will find food anywhere, including swimming prey like rotifers, fish fry and insect larva. Koi on the other hand tend not to identify or attack small prey like that. Their usual foraging method is bottom sifting. And if the pond has no aggregate or mud on the bottom, there will be no natural forage. I think most people would be very surprised by the amazing amount of live, squishy and crunchy food that can be found living in the gravel of a properly maintained Aquascape pond. Sometime if you're ever bored, siphon thirty gallons of water out of that gravel and put it in a plastic sweater box. Let the sediments settle and then look at the writhing life making tracks in the settled sediments. That's Koi food, friends.

How Much Should I Feed My Koi? The Common Mistake of Overfeeding

Can I Underfeed? How Does That Happen? What's It Look Like?

Do You Even NEED to Feed?

What About Feeding in leviathan Aquascape half acre installs?

Why Is There a Temperature "Thing" About Feeding Koi?

What Temperatures Would I Start and Stop Feeding My KOi?

What's "Different" About Feeding Koi in Warm Water?

Does it Matter WHERE I Put The Kois Food? Scattering or Edge? Floating or Sinking?

How Should Koi Food Be Stored and Why Does It Matter?

Let's look at what's IN the feeds?