sick bicolor

bought a bi-color foxface rabbit fish about 2 weeks ago his yellow is turning brown his black has white dots and has a white stripe down it, my nitrites, nitrates, and ammonia read almost perfect zero as far as my testing kit can tell and ph is 8. He eats fine all the rest of my fish are fine...

Hi guys, I am really new here. I have a bicolor angel that developed a dark blue (same color as its back side) streak by its dorsal fin. I have never seen any bicolor like this before. Usually their yellow parts are quite yellow without any blue in them. I think he might be sick. But I am not...