Contrary to popular belief, tuna is NOT healthy for cats, at least not the human kind. Eating too much can cause malnutrition because it lacks proper levels of minerals and vitamins that cats need. Fish in general, when overeaten, can cause a thiamine deficiency. If your cat enjoys any kind of fish food, only allow it a few days a week at most. Stick to chicken and turkey, keep fish and beef at the minimum. Mercury levels are also very high in tuna so purr-haps you should cut back yourself!