Certain types of parasite may cause patches (Costia, Chilodonella, others as well). Sometimes lymphocytosis (due to a virus) may be the problem. Pictures from a fish health book may help you to understand more about it. You may give a try with a proprietary remedy (i.e. commercially available product) which is active on the above-mentioned parasites. Often they contain formalin+malachite green. Dionigi