Hello,
1. What is the best food for coloring up Yellow discus? anything that has no red pigments!(carotenoid pigments) but has yellow pigments and thats the hard part, Most fish foods have red/orange pigments thats the fish will take up. Things to avoid, ...salmon, krill, plankton, spirullina (though green its high in orange pigments),blood worms, and any green veggies usually have orange pigments as well.
2. What is the best *flake* food I can get for discus that (a) won't change yellow discus to red, and (b) is available through Amazon? I've had success with Ocean Nutrition flakes before for my non-Yellow discus, is that ok for yellow discus? You will have a hard time with that as they have too many of the foods to avoid.
I don't usually toot my horn on the blackworms that I sell outside my sponsor board , but the Freeze Dried Blackworms I have here... https://aquaticsuppliers.com/Freeze-...riginal_c6.htm have been used by several of my customers with yellow fish. They won't significantly add pigment to the diet. Its one of the Few foods that I know outside of making your own homemade mix of things like whitefish, chicken and turkey that are generally Yellow Discus Safe.
You can add yellow pigments to the foods with things like yellow squash, the pigment lutein, and if you can find the pigment Carophyll Yellow ( a very strong yellow pigment)
hth,
al