Begin with the spine as a movement line
A fish body is organized around a flexible spine running from the skull to the tail. In a drawing, represent it with one center line. A straight line creates a calm side view. A shallow S curve creates motion, while an extreme bend can make the head and tail feel disconnected.
Build the upper and lower contours around that line. The deepest part of the body is usually closer to the head than the tail. From there, both contours taper toward the narrow tail base. This taper is one of the strongest cues that the fish can move through water.
Separate the head with the gill plate
The head is not a circle attached to an oval. Its top contour usually continues smoothly into the back. Use a curved gill line behind the eye to show where the head ends. The line should follow the rounded volume of the body instead of standing straight like a fence.
Place the eye above the center axis and in front of the gill plate. The mouth belongs near the tip of the snout. Changing these distances changes the species and personality: a short face feels compact, while a long snout feels alert or specialized.
Use the lateral line to wrap patterns
Many fish have a sensory line running along the side. Even when you do not draw it literally, it is a useful guide for stripes, color changes, and scale direction. Let it follow the body axis and taper toward the tail.
Patterns above and below this guide should curve with the body. A stripe that remains perfectly vertical can flatten the form. A stripe that bows slightly toward the head makes the surface feel rounded.
Attach fins at believable roots
The dorsal fin grows from the back, the pelvic and anal fins from the lower body, and the pectoral fin from just behind the gill area. Draw each fin from a short root rather than a single pinched point. The root makes the fin feel connected to muscle beneath the skin.
Fins overlap the body differently. A side fin can cross the silhouette inside the body, while a dorsal fin changes the outside edge. Use that overlap to create depth. Keep the far-side fins smaller or lighter so they do not compete with the near side.
Translate anatomy into style
Realism uses these landmarks with accurate proportion. Cartoon drawing keeps the same relationships but exaggerates one choice: a larger head, wider tail, taller body, or oversized side fin. The anatomy remains useful because it tells you which parts can change without losing the idea of a fish.
Before detail, check four relationships: eye before gill, widest body before midpoint, fins rooted into the body, and a clear taper into the tail. If those relationships work, the drawing can support almost any color or pattern.