Measure the large proportions
Before drawing the contour, compare body length to body height. Then compare head length to total length and tail height to body height. You do not need exact numbers; you need consistent relationships. Mark these distances with short guide ticks.
Locate the widest body section and the narrow tail base. Most fish taper gradually between them. A sudden pinch makes the body look assembled, while no taper makes it feel rigid. Keep the center axis visible until the upper and lower contours agree.
Describe volume with overlap
The near pectoral fin overlaps the body surface. Its root begins behind the gill plate, then the fin projects outward. The far fin may appear as a smaller shape below the belly. This difference in size and overlap tells the viewer which side is closer.
At the tail base, let the body pass into the tail instead of ending at a line. Use a darker seam only where one form truly crosses another. Continuous contours feel organic; too many closed shapes make the fish look like a diagram.
Wrap marks around the body
Scales, spots, and stripes follow the surface. Near the center of the side, marks can appear wider. Toward the top, belly, and tail, compress them. Curving a stripe slightly around the body creates more volume than adding extra shading.
Do not draw every scale. Suggest a few near the shoulder or along a light-to-shadow transition, then allow them to fade. The viewer completes a repeated texture more readily than a drawing can describe it one mark at a time.
Plan underwater light
Light commonly arrives from above, so the upper planes receive more light while the belly and fin roots turn darker. Reflected water can add a cooler light along the underside. Keep this reflected light weaker than the main highlight.
Fish skin is reflective but not chrome. Use one controlled highlight near the upper body and a smaller point in the eye. Large white bands across every edge flatten the form and make the surface look plastic.
Vary hard and soft edges
Use clearer edges around the eye, mouth, and front contour where you want attention. Soften the far fins, the lower belly, or parts of the tail. Edge variety creates depth even on a simple canvas.
Finish by comparing the drawing at full size and thumbnail size. At full size, look for abrupt joins and noisy texture. At thumbnail size, confirm that the head, body, and tail remain distinct and that the eye still leads the viewer.