A series from independent yuri magazine Galette. Ami is in love with Yuu and plans to confess when she's written 100 pages in her diary. In chapter 4, Ami is delighted when Yuu invites her to go to an aquarium. When Yuu asks Ami whether she writes about her in her diary, Ami wonders whether it's a good opportunity to confess, sooner than planned.
They happen to see You, one of their friends from school, also at the aquarium, clearly on a date with a girl. Since You is given very little attention in this series, unless it's a cameo from some other Yatosaki story, it looks like the power of aquariums is such that even minor characters will coincidentally show up there in a lesbian relationship.