Fish have been used as a decorative element for centuries.
Like many archetypal symbols, fish have been used as a decorative element without regard to meaning. How to explain the recurrent use of the carp, say, in English and American Aesthetic movement design of the late Victorian period? Tastemakers of the era were drawn to Japanese design. (Koi are domesticated, ornamental carp. And in Japanese, koi is a homophone for a word that means “affection” or “love.”)
The ubiquitous carp of Japanese design is a symbol of “youth, bravery, perseverance, strength, and self-defense” [Encyclopedia Mythica]. That, and not the fish’s complex Christian symbolism, probably explains its Western popularity during the 1870s and 1880s, extending then into Arts & Crafts and Art Nouveau design.
The fish symbol is ancient and ubiquitous in religion and design, appearing in Greece, Rome, and Scandinavia: among Semitic people; in Buddhism and Hindu mythology; in Egypt, China, and Japan, Oceania and Africa and the Americas. The fish is phallic and fecund yet, carrying the power of the waters and life itself, it is associated with the Mother. There is a universal belief that fish have a kind of pre-knowledge. Fish is also food, sometimes sacred but often a symbol of plenty.