What's this dark spot on my minnow?

Today I bought 8 rosy red minnows for my 30gal tub pond, all look pretty healthy considering the conditions they were in, but one has a black head! Is this piebaldness or ammonia burns? How can I speed the healing process of they are ammonia burns? They are fed on live mosquito larvae, soaked goldfish flake, and any insects that fall into their pond. They are the only fishes in the tub.