Have you had her checked out for hyperthyroidism? As her appetite is poor this might not be the case here, but you never know.
Also, my vet advises to steer clear of fish-based cat foods, unless you know the iodine content. He recommends a non-fish diet for all cats, not just those with thyroid issues - when did you last see a cat catch a fish? Something about prolonged exposure to excessive iodine in the diet can predispose some cats to hyperthyroidism in later life.