Add basic features.
Add the eye, nostril openings, and mouth. Note that bony fish tend to have lower jaws that jut out past the upper jaws, the opposite of sharks. For a tuna in profile, add a vertical curve to represent the gill cover, and a lateral line from head to tail. This line represents the swim bladder, which helps the fish orient itself and maintain depth in the water.
Use your reference photos to help you add fins. On a tuna, the pectoral fins directly behind the gill covers are like the arms. Draw the ventral fin on the chest area right below the pectoral fin, and the long, spiny first dorsal fin on the top of the tuna. Then draw the second dorsal fin, which looks like the top half of a boomerang. Then add the bottom half of that boomerang, the anal fin, on the underside.
Tunas also have a ridge of small triangular fins that go from those dorsal fins to the tail, so be sure to draw those. Finally, draw the tail, which is shaped like a crescent moon with symmetrical lobes.
If you’re sketching a tropical fish like the Moorish idol, you have fewer features to worry about. To build on the basic circular shape of the idol’s body, Braun suggests adding some loose, rounded triangles to the top and bottom, adding two triangles for the tail, and, of course, a small oval for the eye.