Start with the body flow
Imagine a line running from the head through the center of the body to the tail. Patterns can cross this flow, follow it, or gather around one point, but they should react to it. Sketch the flow lightly before adding any decorative marks.
Leave the head, tail base, or belly partly quiet. Empty areas give patterned areas room to register. Covering every surface with equal detail makes the fish harder to read and removes a useful sense of rhythm.
Use stripes to show volume
Vertical bands feel stable and can make a long fish feel taller. Curve each band slightly around the body and narrow it near the upper and lower edges. Diagonal bands create movement, especially when they lean toward the tail.
Vary the width and spacing. A broad stripe followed by two narrow stripes feels designed; equal bands can feel mechanical. Break one stripe around the eye or fin root to create a focal area.
Organize spots into groups
Random does not mean evenly scattered. Place spots in clusters of different sizes, then leave gaps between clusters. Larger spots can sit near the widest body area, while smaller spots taper toward the tail.
Repeat one spot color in the eye or fin to connect the design. Keep the highest contrast spots near the focal point and reduce contrast elsewhere. This lets the viewer look through the pattern instead of being stopped by every mark.
Suggest scales instead of counting them
Use overlapping arcs or short diamonds in one or two areas. Align rows with the body curve and compress them near the tail. Fade the marks before they reach every edge so the texture remains a suggestion.
A scale field works best where light changes or where the body is broad enough to show repetition. On small digital drawings, a few directional marks often read better than a complete grid.
Combine patterns with a clear hierarchy
If you combine stripes and spots, choose one as the main system. For example, use two broad body bands and place a few spots only inside the darker band. The second pattern then enriches the first instead of competing with it.
Review the fish as a silhouette, then as a color block, then with pattern. Each layer should remain understandable. If the pattern hides the body divisions, remove marks near the gill, tail base, and fin roots.