shizuku no oto
Shizuku no Oto (also known as Sound of Drop -fall into poison-) is a game by LYCORIS.
In Tokyo's Manten Aquarium, a place shrouded in urban legends like blood-red waters on full moon nights and man-faced fish, junior high students Mayu and Himeno visit to investigate. Their fun investigation takes a dark turn when they spot a glimpse of Mayu's missing younger sister, Mari. As they pursue Mari's shadow, fate intervenes, leaving Mayu alone and lost within the eerie, crimson-tinted aquarium.