As I recall from early discussions with our vet, one of the specific problems with cat food for dogs was the level of ash. Generally, the types of nutrients cats and dogs need are different - it's not so much the types of food, but how they're balanced.
As long as the dog's main source of nutrients isn't the cat food, you're probably ok.
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Dogs love love fish. One of my dogs' favourite commercial treats is dried and rolled salmon skin. Their favourite treat when they visit the hamburgers' is grilled salmon or halibut.
The vet says fish oil helps keep their coats lustrous.
The downside is seriously stinky dog breath