Hey, Aquarist!(2023)

You, Aquarist

Tetsuo is a tough looking boy who has hair dyed bright red and wears flashy clothes. He's strong, but is trying to avoid needless fighting. One day, he found a plastic bag with two goldfish inside abandoned in a trash box on a summer festival day. He had a trauma...of the day he killed goldfish by accident. He tried to release them into a river, but a girl stopped him--a beautiful girl who knows so much about goldfish. She told Tetsuo that releasing them means killing them, that goldfish were meant to be kept by humans. When Tetsuo decided to keep the goldfish, she beamed and came to his apartment room to teach him how. They weren't aware that another cute girl, Mayu, was watching them...she had something to tell Tetsuo about fish too.

(Source: Kadokawa)