Nettaigyo wa Yuki ni Kogareru. Manga by Makoto Hagino.
After her father traveled overseas for work, Konatsu Amano moved to Iyo-Nagahama to live with her aunt. On the way to her aunt's house, she gets dragged into her new high school's aquarium open house. It is there where she meets Koyuki Honami, a pretty upperclassman. After encountering each other a few more times, the girls quickly become infatuated with one another. In an attempt to get closer to Koyuki, Konatsu joins the aquarium club, which previously had Koyuki as the only member.