Even the best cat food brands are trying to make a profit. People are familiar with salmon and believe it to be healthy, plus cats really like fish, so it sells really well. Lots of cat foods are full of unhealthy ingredients.
It's kind of a judgement call. The general rule is once in a while/in moderation is fine. Personally I only feed fish as treats, but I think many people on this forum do have the occasional fish flavor in their rotation. It depends on the cat, too. Cats with kidney issues should particularly avoid fish, for example.
Yup. My oldest cat does that as well with specific flavors. Its started feeling like trying to get a picky kid to eat. I told him yesterday "You can't have your kibble until you eat at least a few bites of the canned food." It seemed like he understood since he started eating the canned right away.
I'm overseas so I have access to a lot of British brands. This one is Waitrose, which is a high-end British supermarket similar to a Whole Foods in the US, or a Dean and Deluca (another British brand, if you've heard of them). I have no idea if it makes a difference if the tuna is off-brand or name-brand in terms of quality. I stayed away from Starkist and other big US brands because of their poor record with environmental groups and sustainable fishing advocates.
Good luck! Meant to add: Be sure to buy tuna packed in spring water and not oil, brine, or anything else. You probably already know that but just in case.
alibali
Fish flavoured complete foods are ok in my eyes. I have a fish addicted boy (who's also had crystals). It's been a fight to get him to eat good food, and I'd far rather feed a fish flavoured high quality canned food than give him a lower quality food that's fish free. Sometimes you have to make compromises - the best food in the world is useless unless it gets into the cat!
juleska
Go very carefully with the tuna. If possible, I'd try to transition to a different fish - tuna really is the most problematic fish out there. You might do better with something like mackerel, sardine or salmon. You could also try bonito flakes as a topper over chicken or other meat to see if that would tempt Luna to broaden her palate a little 
I'm conscientious of this. She is getting a really high quality dry food, though, so I'm not feeding her *only* tuna. I agree with you, however, that reliance on a single form of protein isn't good. I've tried her on salmon and she wouldn't touch it. I've tried chicken and she nibbles but never more than a bite or two. I've tried other types of fresh fish that I baked and the most she did was lick it. Egg, wouldn't touch. Crab, not interested. The ONLY thing she'll eat is the tuna and any kind of wet cat food....every single brand of which gives her bloody diarrhea after just one helping.Firstly, I'd like to draw attention to this article.[article="31650"]Can I Feed My Cat A Fish Based Or Fish Flavored Diet [/article]
The problem with straight tuna (aside from mercury levels etc) is that it is both deficient in vitamin E and depletes B1 levels, potentially leaving a cat with two MAJOR deficiencies that can make it very ill. A teaspoon of tuna as an occasional treat is fine, fish flavoured complete and balanced commercial foods are ok (as the overall recipe will compensate for these issues), but a diet based around straight tuna is a recipe for disaster in the long run.
@alibali Fish flavoured complete foods are ok in my eyes. I have a fish addicted boy (who's also had crystals). It's been a fight to get him to eat good food, and I'd far rather feed a fish flavoured high quality canned food than give him a lower quality food that's fish free. Sometimes you have to make compromises - the best food in the world is useless unless it gets into the cat!
@juleska Go very carefully with the tuna. If possible, I'd try to transition to a different fish - tuna really is the most problematic fish out there. You might do better with something like mackerel, sardine or salmon. You could also try bonito flakes as a topper over chicken or other meat to see if that would tempt Luna to broaden her palate a little
white shadow
That article about hyperthyroidism is interesting - we were just at the vet yesterday and they are testing my fish-lover for it!! (He was recently diagnosed as epileptic and apparently hyperthyroidism can cause seizures!)
