It is one of the top three most common feline food allergens. Symptoms include a loss of appetite seizures and possibly death. If you want to feed a fish-based food as a treat please limit it to once or twice a week.
But not all food is safe. ORIJEN Dry Cat Food Six Fish Biologically Appropriate. Another problem for cat owners is that cat food that has a lot of fish in it frankly smells horrible and can linger. Basically there are toxins in fish flavored cat foods that are contributing to the epidemic of feline hyperthyroidism.