Use your reference photos to help you to 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.
Tuna also have a ridge of small triangular fins that go from those dorsal fins to the tail, so make sure that you 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.