Tuna is not particularly good for cats or humans, regardless of how it's packed, due to it high mercury content.
Sardines are better but omega 3 fatty acids from them can destroy vitamin E in cats, so when you give her fish high in omega 3 you should look into vitamin E supplementation. Or just don't give her any fish and feed her high quality canned cat food, so you won't have any worries about properly balancing her vitamin intake. Sardines also contain more phosphorus than calcium so even if you do supplement the correct amount of vitamin E, they should never exceed 10-20 percent of your cat's food intake, provided that the remaining 80-90 percent is correctly balanced cat food.
If you want to feed your cat well high quality canned cat food or raw food balanced to meet cat's nutritional needs is the way to go.
I am learning lots and lots since adopting these 2 kittens. One thing that I have obsessed over is diet. Believe it or not tuna is not good for cats. It is not a natural part of their diet either (cats ~ land~ hate water. Tuna~ water~ doesn't thrive on land lol)
Also, cat will become addicted to tuna and turn away any other food options. This leads to vitamin issues and general health.
RAW diets, homemade cooked diets are all great but you have to know what your doing. Make sure to research it a lot before diving in. You don't want kittie missing anything in its diet.
The best thing to do is start researching cat foods. Everyone has a favorite but do your own homework. Look for a grain free canned food if RAW or homemade diets aren't good for your lifestyle.
If you want to treat kitty, try baking an extra SMALL serving of whatever meat you are using for dinner. Just cook it on the side without spices or gravy. I do this often when making anything with boneless chicken breast. Sasha loves it as I shred it and give it to her as a treat.
Look for beef as well. So many folks think fish and chicken for a cat (I was really guilty of this myself). Beef has taurine in it and this is something cats need. It can be found in beef. So toss the whole chicken or fish only thought away and give her some beef too =D
All meats have Taurine, but it's the concentrations levels that vary. Rabbit is one of the most highly concentrated. Just as an FYI, they do sell frozen sardines, anchovies, salmon, tuna, etc. with no additives or seasonings when you do want to feed fish, but fish is high in fats for cats so it should be fed at a minimum. You also don't want to shy away from entrails (hearts, gizzards, liver (although liver should be fed at a minimum as well), tongue, brains, lungs, etc) you can always lightly cook them for your cat or feed them raw. These are very rich in wonderful nutrients.