Dark or tough skin is often removed from fish fillets before they are cooked. Use a sharp knife with a long flexible blade to remove the skin. Here, a turbot fillet is shown.
Place the fillet on the work surface, skin side down, tail end toward you. Make a small cut at the tail end to separate the skin from the flesh.
Grasp the skin with the fingers of one hand (dipping them in salt or using a cloth if they slip). Hold the knife between the skin and the flesh, with the edge against the skin and the blade almost parallel to it. Work away from you with a sawing motion to remove the flesh, at the same time holding the skin beneath taut with your other hand.