Mine have sardines in a couple of feeds a week too and their coats are lush but here is an oil you can buy called moult oil which is supposed to give them good coats and you use a teaspoon or so a day.
Mine have seaweed & Apple cider vinegar in their feed for their coats, makes them really shiny & silky plus no creepies!
Young dog also gets a Bantam egg in her supper just to tempt her to eat & also use up the over supply of eggs from the banties!! Plus very cheap!