Did you ever think that fish could have an allergic reaction to something you feed them, or a plant in the tank? Like, Humans, cats and dogs i know can have allergies.. but what about a fish?
Not sure about allergic reations becuase some foods try to imitate natural foods, however it's hard to do. Now I would not doubt that they may have "allergies" or sensitivities to their water types, like hard water and things like that.