You could try vitamins. Omega Caps (I order them from healthypets.com) is a fish oil blend that I just puncture the capsule and squirt into Smudge's canned food. He likes the taste.

A good brand of tuna for cats is Solid Gold. It's got all the extras that a cat needs when eating tuna, which can otherwise strip out a lot of essential nutrients from a cat's system. It's in a purple can with white stars.

Love, Columbine