My 7 yr old Scotty has an elevated alkline phosphatase (1200+). It was suggested I put her on Science Diet ID?

Due to the filler ingredients in S/D, I have instead put her on Wellness Whitefish and Sweet Potato Diet. She also has chronic skin problems which I treat only with omega oils, antihistamines and special shampoos. It's only been 2 weeks with no improvement. Vet thinks it may be "borderline Cushings". Any dietary or other suggestions?