If you are looking for nutritious food, made from fresh fish with just a bit of veggies/fruit added - no added wheat or unnecessary carbs - take a look at Orijen's Six Fish cat kibble. No...seriously! Forget for a moment it says "cat food" on the bag and just read the ingredients and how it is made. Folk over here use it to raise discus (I feed mine wild salmon), but I whizz the kibble down and feed it to my smaller fish. It is expensive as cat food but actually Lb/£ it works out better than half decent fish food. (Also forget the class action against the company, probably bogus)
I have been feeding my Koi a grain free diet of Origen Puppy Chow for last 15 years or so. Its made fresh right here in Alberta Canada and is a high quality puppy chow. My two oldest koi are about 27 years old or so, and the rest of my koi are all doing just fine. I feed them different sizes, large breed puppy for the big koi and small kibble for the rest. My fish live outside year round and they need the high fat diet. Fish love that food!
I also have heard of many people feeding them the cat food as well with good results.
The problem with high priced Koi food is that it sits on the shelf at pet stores for a much longer time than the dog food, and so the dog food is very fresh. Some of that high priced koi food is way over priced and a small bag costs about 100 Canadian dollars and my koi eat that in less than on month, where as the dog food you get a really big bag for about 80 dollars that feeds my puppies and my koi all summer!
Koi food is way too expensive and way over priced, and lots of it is old, still sitting on shelf at store in many cases....
Then Koi are really just Carp scavengers and eat many different food that they would find in a natural pond bottom, including dead and rotten animal's that may have met their demise in nature, and sunk to pond bottom, so feeding them them over priced koi food seems senseless.