Koi Uta Manga恋唄; Love Song (KAMEI Yogorouta)
This story comes from a collection of fantasy oneshots with a dash of animal-ear flavor. In the other stories (not available here): an onnagata inspires a young boy to get in touch with his inner woman, a family of foxes teach a miserable young tailor to cure leather, a cat is trapped in a pretty intense love triangle, a bumbling newspaper club member is delegated to photography club duties, and a boy with a history of sexual abuse falls for the painter he wants to cover those memories up.
*We'll only be doing this oneshot from this series.
Chapter 6: Love's Fountain of Prayers
Sasa is a merman whose curious about the world of humans. When a human, Yuuzutsu told Sasa that he loves him, Sasa decided to go to the local apothecary, Sou, in order to find a way in order to get legs. Sou relectutantly grant Sasa his wish by replacing his heart with a "love's soul" on the condition that Sasa must procure true love with his partner or he will die.