I've read that feeding cats real tuna is very bad for their health because it's high in polyunsaturated fat which cats have trouble metabolizing. Tuna can rob a cat's body of vitamin E, leading to a painful disease called steatitis, or Yellow Fat Disease, in which the skin becomes unbearably tender. I copied this from a book that I have about cats. My parents learned the hard way because they fed their cat real tuna and also real chicken liver every day because she was a very picky eater. She didn't live as long as she could have.

If your vet thinks that your cat needs the extra oils then I'd just supplement his diet with some omega 3 fish oil instead. Good luck.