Basically you feed them wheatgerm and garlic in late summer/fall then again in the spring.
Moving on into the summer months i;e from May onwards you can strart feeding with wheatgerm and garlic plus summer growth feeds .
You then can feed various high protien feeds that have special properties "say for colour health and virbance" NTLab's that you can feed your koi during this period readying them for the winter ahead .
Look into feeding your koi freeze dried Silkworm, Mealworm, Shrimp every few feeds during the summer, fresh foods such as Orange Lemon etc are also a welcome treat
At the moment I am feeding my own koi with wheat germ and garlic and a mix of both it and a high protien Japanses mix of foods ready to move over for the summer
I hope this helps
Dave