From what Brett has said about Rangen feeds they provide a well balanced diet designed specifically for koi/carp fish. That would imply a diet designed to promote a healthy fish that maintains good body. I would still be feeding Rangen if I knew why my fish stopped eating it. Which kinda/sorta brings me to my next thought re your feeding practices. What is the result to the development of your show fish from the Hi Silk and Izeki, shrimp, and endive lettuce diet that you have found beneficial to their show appearance. From reading about your prize winning koi you must be doing something right.
Which also leads to the question...why do we feed the higher cost feeds and what do we expect out of them? Is it marketing? Is it because the big show winners, as a generalization, feed those types of imported feeds? And then there is the corn/grain based issue. Long threads devoted to the evils or benefits of feeds based on grains.
And then back to the beginning of this thread...are those who regularly use the feeds they indicated they use seeing the benefits they expected? What lead them to the feeds they are using at present?
Steve