Several parts combine into one character.

Pictograph: a side view of a fish — head on top, scales (the middle 田-like grid) in the body, tail fin at bottom. Oracle bone forms are even more recognizably fish-shaped. Three forms: 繁體 魚 / 新字体 魚 / 简体 鱼 (the simplified form replaced the bottom four-dot tail with a single line).

Chinese New Year cultural pun: The Mandarin word 鱼 (yú, fish) is a homophone with 余 (yú, surplus / leftover). The phrase 年年有余 (niánnián yǒu yú, "may you have surplus every year") is a classic New Year wish, and is why fish appear on dinner tables for Chinese New Year — the dish embodies the wish for prosperity. This pun is invisible to non-Chinese speakers but central to the cultural meaning of fish in East Asian dinners.

魚 / 鱼 is one of the most productive radicals for fish-species names: 鯉 / 鲤 (carp), 鯨 / 鲸 (whale), 鮭 / 鲑 (salmon), 鯖 / 鲭 (mackerel), 鮨 (sushi-grade fish, Japanese only). Walk through any Japanese fish market and you'll see dozens of 魚-radical characters.

Mandarin: yú, rising 2nd tone (simplified 鱼). 鱼 (yú, fish), 钓鱼 (diàoyú, fishing — also slang for "catfishing" online), 鱼肉 (yúròu, fish flesh), 金鱼 (jīnyú, goldfish), 章鱼 (zhāngyú, octopus). The Internet slang 摸鱼 (mōyú, "stroke the fish" → slack off at work) is everywhere on Chinese social media.

Japanese: on-reading ギョ (gyo) for compounds — 魚類 (gyorui, fish category / fish kind), 熱帯魚 (nettaigyo, tropical fish), 金魚 (kingyo, goldfish — note kun-on hybrid), 漁業 (gyogyō, fishing industry — using 漁 = water + fish). Kun-reading さかな (sakana) is the everyday word — お魚 (osakana, fish — polite, often refers to fish as food at dinner table). The reading うお (uo) is older / more poetic — 魚市場 (uoichiba, fish market).

The Japanese sushi/sashimi vocabulary is built on this character — 寿司 (sushi), 刺身 (sashimi), and the entire menu of Japanese 魚 (sakana) types.

Memory aid: head + scaled body + tail = the fish, drawn whole from the side.