There are so many kanji for fish. So many that they sell mugs that have all of them printed on the side with the readings in hiragana – a cheat sheet / novelty item.
For the most part, they’re written in katakana, but you will find them written in kanji every now and then. My JTE, who has moved on to a different school this year, always used this kanji when he taught the passive voice – “This kanji is called ____ in Japanese.” He wrote a bunch of difficult kanji on the board, which I guess was fun for the kids to try and guess. He mostly used different characters for fish.
I always thought this was the coolest of the ones he wrote on the board. Fish on the left, capital on the right, which gives us whale – the biggest of all things that swim in the water and appear not to be mammals.