You, Aquarist

A delinquent boy was out at a festival when he finds a discarded goldfish. Recalling his childhood experience of buying a goldfish from a festival stall and bringing it home only to have it die, he tries to make up for it by releasing the goldfish into the river. However, a mysterious girl stops him and tells him that it is not allowed to let out goldfish into the wild, and he learns how to keep goldfish at home and becomes a proper "aquarist."