I got an Iams booklet with my kitten when I got him three weeks ago and it says:
"Never feed the following people foods to your new cat:
-Tuna - low in calcium and too high in phosphorus; feeding exclusively may cause a vitamin E deficiency (yellow-fat disease) or rubber jaw, a form of bone demineralization."
So, what I gather is that tuna is okay every-now-and-then but if that's all you feed your cat, it's bad news.
If a cat only eats fish (any kind) it will get sick but to give fish once in a while is OK and won´t harm the cat. Give tuna as a special treat sometimes. The danger is when the cat lives only on fish.
Tuna is a very stong fish but feeding it as a treat won't hurt or if your worried or want to give an extra fishy treat you could bake a mild white fish such as talapia. Mix it in with their dry food or some white rice that is good for their digestive tract. The omega fatty acids in the fish are very goos for their coat too.