My dog has a food allergy that causes yeast infections in her ears...?

well, she has been to the vet so no need for anyone to tell me to take her there. I now feed her purina pro plan for sensitive skin and stomach that's salmon based. We think she is allergic to any land dwellers because we tried chicken based food, lamb based food, beef based food and duck based food. The stuff she's on now works great. My question is: could we feed her cat food that is fish based? The reason I am asking is because the stuff she's on now is a bit expensive for us right now. Obviously she will get what she's on now regardless. I was just wondering if it would be ok to feed her the cat food.