Right now I'm just doing some googling and found an article by Sandy Yosha,
who has no involvement with fish food makers. She was our vet for the KHA's
paid by the AKCA, she now works for a very big vet. clinic in Florida doing
only fish.

From this website:
http://www.nda.agric.za/docs/AAPS/Ar...koi%20husb.pdf

This small insert:
Nutritional Diseases- (Review the Nutrition section). Nutritional diseases
fall into two categories: deficiency or toxicity. The most common
deficiency known to date is vitamin C, which can cause
crooked backs, bone deformities, bleeding, abnormal wound healing and many
other problems. Vitamin C deficiency compromises the immune system so that
the koi are more susceptible to other diseases.
With the advent of stabilized vitamin C in the food (a synthetic vitamin C
that is much more stable than natural vitamin C), vitamin C deficiency is
less common. However, much less is known about the other
vitamin deficiencies in koi and they might be suspected in cases where the
cause of disease is not immediately obvious. The signs of deficiency or
toxicity are complex and subtle, and are covered in the
supplemental textbook for this course. Also review the AKCA Guide to Koi
Nutrition and the nutrition sections of this course.

The KHA is not likely to be dealing with vitamin deficiency if the fish are
eating a quality diet, but if they are not, it will be one of the first
changes in husbandry that the KHA should suggest.

BoldCatfish and trout chows are not appropriate feed for koi.Bold

Many will debate the quality of koi feeds, but there are many good choices
that are commercially available. All feeds are not necessarily appropriate
for all life stages or for all seasons. One fact is certain, good quality
feed will contain stabilized or synthetic vitamin C. Review the nutrition
sections for additional information. In order to maintain nutritional
value, check the shelf life of the feed, the date of milling or freshness,
buy in small quantity and keep the feed dry and cool. Freezing pelleted
feed is not recommended, but short term freezing of pelleted feed
containing a medicine can be done since it is a small quantity....
----
I met Sandy, really respect her knowledge, I know she doesn't work for the
pet food industry. Her write up, and there is far more to it than the
above, including pictures, is well worth the read. As it is more than just
nutrition. Unless I hear back for the KHA board on a better article, you
all can google as easily as I can. :-) I'm convinced, and like I said, an
owner can feed their fish, dog, cat whatever they want, IMHO. I just don't
think it is prudent to be telling other people who ask, "What is best?" To
go in that direction. ~ jan