You're going to get several answers that trout and cat food is fine. But if
you were to do a little reseach on this you'd find that not only is koi
food better for koi, it is better for your pond and filter.

Out of curiosity, and speaking of research, are you aware of anything proving
this to be true? You know, single blind study of koi versus cat foot, or such?
C//

On the fish or the filter?

Within our club we have a member keeping koi in a large aquarium while the
pond is being built. He was using a medium priced koi food, and had
troubles with water quality. He switched to Manda Fu and it cleared up.
Then he ran out, went back to the old stuff and had the same troubles. The
extra price was worth the savings in time to him, he's since reordered the
Manda Fu.

Otherwise, I've read many articles on this in both water gardening & KOI
USA, written by Vets and other knowledgable people. The latest one was
about how koi lose color after coming from Japan to the states. The single
factor they could point too, cheap or incorrect food.

Then we have Gale's personal experience.

Personally, I can't understand why people would spend bukoo bucks on their
ponds & fish and not shell out a little extra for proper food (and test
kits). My dog's food cost more in 1 month than I ever begin to spend on the
koi in 3 to 6 months. Like I said though, to each their own. I'm hoping my
koi live a long, long time. My water quality is excellent and my fish are
healthy, so I'll stick with what I'm doing. ;o)

If others are having same with the cheap stuff, I have no beef with that,
but over the long haul, Gale's experience could happen. I'm not here to
debate with those happily feeding what they're feeding, I'm just imparting
what I've learned over the last 10 years of serious study on all things koi
for the OP who asked. Let's not make it into an us verses them
debate. ~ jan

~Power to the Porg, Flow On!~