Unless one of your cats has a medical condition that requires a special diet, I see no problem with giving them a little bit of tuna fish now and then.
Tuna has taken a bit of a bad rap in the world of cats. For a while, some people believed that tuna was linked to urinary crystals because of its high magnesium and phosphorus content. As well, tuna may contain high levels of mercury, which is bad for both cats and humans.
In fact, recent studies have shown that excessive levels of magnesium and phosphorus in the diet are not likely to cause urinary crystals. The real culprit is an abnormally low level of acidity in the urine (click here for more information about urinary crystals in cats). And if tuna is used sparingly as a treat then the risk of mercury poisoning is extremely low.
Cats absolutely cannot survive on an exclusive diet of tuna. However, if tuna comprises only a small portion of their food intake, it is unlikely to cause problems. Thats a good thing. The cats I know would consider me very cruel indeed if I were to open a can of tuna and not give them any.