Ok, it's our game and it's self-promotion, but I strongly encourage you to take a look to our game: https://buendiagames.itch.io/the-only-man-and-the-sea
The artist (elenoideart in Instagram) is SUPER NICE and did a wonderful work with the fish animations. Also, the art is free under a Creative Commons license, so if you dig inside the GitHub repo (https://github.com/BuendiaGames/OnlyOneFish) you can find the spritesheets. You may learn how to animate fishes (?) or any other pixel art trick.
But you game looks also very nice! :D Keep improving your art!