Tuna is high in phosphorous and low in calcium, which is not a good thing (There is a specific ratio of phosphorous/calcium amount that should be maintaned). It may cause vitamine E deficiency, yellow-fat disease (a disease of the liver) and may increase susceptibility to osteoporosis.
Other than that, tuna for human consumption are high in sodium, which is also bad for cats.
The other problem about tuna and most big fish is the mercury. They have much more mercury than the smaller fish. (This even poses danger for humans; especially children and pregnant women shouldn't consume large amounts of tuna or other big, prey fish such as shark).
As for the fishy foods made especially for cats, I don't give my 3 cats any fishy food, since fish used for pet consumption are preserved with the chemical ethoqxyquin. It's even used in much more amounts which is let for human consumption. You may not see the preservative ethoxyquin on the label, but they are preserved with it even before they come to the pet-food manufacturing company.
Mercury and preservatives such as ethoqyquin, BHA and BHT, even in small amounts, can add up in the body and may cause cancer, immune system diseases and other diseases as the cat age.
I occasionally give tuna to my cats. Once a month, only 1 can for 3 cats.