I don't know if any of this will help or not, but my old dog Raven was actually allergic to fish. It made him itch all over, especially his feet. His was getting really pink and irritated and he had a lot of dandruff. My vet gave him supplements that consisted mainly of salmon oil. His stomach turned bright red and he started itching like crazy. Took all fish products out of his diet and the rash and dandruff cleared up.
My girl Scarab has a rash/ dark discoloration in her groin area right now. I tried to upload pictures but I'm technologically challenged and it says they are too big. My vet took a skin scraping and found yeast. She put Scarab on a topical mousse (Phytovet CK) which is mainly chlorhexedine. After a week the discoloration was spreading to her neck. I went over everything I was feeding her and the only problem I could come up with was one of the foods had brewers yeast in it. Although I had been feeding the food to her semi regularly, I had recently been using it as my go to dry dogfood. After only a couple of days off the food, I started to see an improvement. I can't swear that that was the problem, since I chose to continue the topical, but thats what I'm going with. I wish I could post a picture.