Carved fish scales can be abstract, real-life or a combination of both. Fish species differ greatly. Some have almost no scales, and some are nothing but scales. Sometimes woodcarvers use forced perspective and carve them larger-than-life. Factors like these make fish and scale carving open to interpretation, but there are several types of hand tools that can simplify this type of carving.
The bent spoon is a versatile woodworking tool with a sharp, semi-circle tip. It is typically used for gouging, removing material or making smooth, round channels. It is often overlooked for detail, but if you stand the bent spoon vertically, and tap the end of it with a mallet, it will create a perfect semi-circle resembling a fish scale. Tap scales randomly, moving about the body of the fish to give it life-like qualities. Tap hard for deep, large scales on the body of the fish and lightly for smaller scales around the mouth and tail. There are lots of different sizes of bent spoon tools. Intermix them for best results.
Other overlooked tools that you probably already have in your toolbox can be used to carve scales. Drill bits -- especially Forstner type drill bits that have circular cutting heads -- make great scale carving tools. Hold the fish at an angle and spin them with your fingers for light scales, or install them into a drill press or cordless drill and lightly touch the spinning bit to the fish. You can tap the bits with a mallet or the end of a screwdriver for light accents or deep cuts.
Punches and Nail Sets
For multiple smaller scales on fish, try using a large nail-set. These tools often have a small dimple in the center that can add relief or depth to a carved scale. They are also round-tipped so you will need to hold them at an angle to get the semi-circle shape. Punches come in all sizes, they are heavy steel and will dent or cut wood with ease. Use them like nail-sets by tapping them randomly around the body of the fish. Mix them up with other tools to prevent any static or benign looking fish.
Traditional fish scales can also be carved with any type of knife. Woodcarvers use what is called a "bench knife." These typically have a slender tapered end to fit into tight spaces, but they work well for intricate carving as well. Other bench knives have angled tips, curved or straight. Carving scales individually with a knife takes time. Typically, knives are used by professionals with a goal in mind. They don't normally work randomly and plan each move ahead of time. Try mixing bent spoons, drill bits, and punches to create scales and then use knives to surgically detail them by adding relief and depth.