Here in the US we are limited to alot of Japanese brand koi food that claim to clean up shiroji. I there was one I would like to try in Momotaro's white label koi food which we can not get here.
I know there are some foods now claiming to bring the "natural" diet back to koi with things found in their natural habitat. Which I still kinda question, since Nishikigoi have been domesticated for the last 100 years or so...But I still too each is his own when it comes to raising their fish.
What do I use to maintain the shiroji or improve the shiroji on my fish? I use a common diet of Sweet Potato year as my staple and Hi-Silk/paste in the summer growing months. Plus alot of water changes to go with my feeding routine.