The entire thing is from Shoujo Friend Suspense & Horror Winter of '96

Edogawa Aquarium—This is an aquarium that keeps an extremely rare mermaid. Nina, the star mermaid of the aquarium, is head over heels in love with her keeper (and adoptive father), Tobio Tobimatsu! With the seals that live with them watching over them, what will become of the love between the mermaid and the human? A bizarre aquarium comedy by Osamu Edogawa begins!! (Source: Comic Ambre, translated)

*"We will never again emerge onto the surface. In this world, you and I are all humanity."* Early Showa era, Tokyo. A beautiful young man, Minoura, has a lover named Hatsuyo, who he is engaged to marry. One day, Hatsuyo is murdered by an unknown assailant. Minoura speculates that Moroto Michio, who had feelings for him in their school days, may have killed her out of jealousy. Moroto is a perfect man, smart and good looking, but he is only interested in Minoura. For the sake of his beloved, Moroto devotes himself to cooperate with his detective work, but...?

Based on Rampo Edogawa's detective story "Kuro Tokage".