Blocking in the posture, proportions, and negative space body angles are the first steps in drawing a fish. You may see a parallel here to drawing birds, mammals, and everything else- it is the same system, just a different subject. Before you add any detail, block in these elements. If you add color with watercolor as in this demonstration, start with lighter values and work your way into the darks.
Click on the first image to start a step-by-step slide show.
Start with the posture line through the central axis of the fish. Add a vertical line to help you compare the proportions of the fish. How long is it compared to how deep is the body? Block in the mass of the body. This fish is fairly round in profile, others are longer. Draw lines to mark how much of the body is head and tail. These proportions will be different on each species. Visualize the negative shapes above and below the face and tail. Carve in these negative shape angles. Note here that the forehead angle is longer than the area below the mouth so that the mouth is below the center line. Expect different mouth positions on different species. Now we will place dots to indicate the beginning and end positions of each of the fins. There are two things to note as you place these dots. The first is the distance along the outer edge of the fish shape or how far back each point sits. The second detail to notice to help you place the fins is to compare where each fin intersects the body while comparing the top and the bottom fin positions. Does the ventral fin start in front of or behind the dorsal fin? You do not need to draw in the pervious visualization lines to place the dots but carefully analyze the fin positions before continuing. If you need to move a fin, now is the time to do it. It is easy to move a dot, hard to move a fin. Now place the fins based on the locations of your guide dots. Block in the back edge of the gill plate and the proportions of the eye. Note that this eye is below the centerline. Locate the pectoral fin behind the gill plate. Draw over your light guidelines with deliberate smooth lines. Refine your linework, erasing and replacing lines with more accurate and nuanced lines. Start by painting the shadows with Daniel Smith Shadow Violet mixed with a little gray mud from your palette. Add a pale blush of magenta to suggest iridescence. Paint the light yellow ring with Hansa Yellow Light. Let the first coat of paint dry and add details with a deeper orange-yellow. Enhance the feeling of iridescence with a blush of Phthalo Yellow-Green. With a jittery brush stroke, add a black line along the lateral line. Continue to add black across the body. Keep the thickness and spacing of the lines consistent with what you see on the fish. Add the other black accents on the back, face, and tail. With a diluted mixture of the black paint, continue the black lines through the pectoral fin. The lighter values makes the fin look transparent. Being careful not to smudge the dark lines, add more gray accents and details in the white part of the fish. It would have been easier to have done this before adding the black but sometimes you only figure these things out as the drawing moves along. The order here is because of a mistake, not an intentional sequence of steps. Add a yellow accent by the pectoral fin. Use a white gel pen to add subtle highlights and to extend the white lines into the black along the back. Switch back and forth between this and the previous slide a few times to see the impact of the gel pen. Use colored pencils to make an X grid across the back of the fish, suggesting scales. Don’t go overboard on this, a subtle suggestion is better than pushing the pencil too far. Here I also crisp up the edges with a sharp pencil.