WARNING: The following contains significant spoilers for Season 1 of Kageki Shojo!!, currently available on Funimation.
Kageki Shojo!! is the story of Sarasa Watanabe, a Kabuki theatre fan who moves to a prestigious school to join the Kouka theatre troupe, loosely inspired by the real-life all-female musical company Takarazuka Revue. When the positive and ambitious Sarasa decides to audition for the role of Tybalt in a graduation ceremony performance of a scene from Romeo and Juliet, she struggles to get into character when she realizes that she has never hated or had a grudge against anyone. That is until she remembers being somewhat jealous of her childhood friend Akiya Shirakawa inheriting the coveted role of Sukeroku when Kabuki theatre generally excludes female performers.
Though Sarasa doesn't hate Akiya, her frustration with him grew when she overheard him agreeing to ask to be her boyfriend so that he and his Kabuki family could keep in touch with her when she moved away. Not only this, but Akiya agreed to this plan under threat of not inheriting the Sukeroku role. Sarasa met Akiya at their agreed aquarium date regardless and asked to be his girlfriend first. Remembering the injustice of losing her true passion for Kabuki, and likely the indignation of almost being asked out with an ulterior motive, Sarasa uses the memory of the aquarium to motivate her to embody a jealous, wistful Tybalt.
The aquarium scene itself is filled with detail. The scene begins in Episode 12 with a shot of rising bubbles followed by a swarm of jellyfish. Each one is distinct, moving in various directions and with different levels of bioluminescence. This is followed by a static shot of eels emerging from the sand, each casting different-sized shadows like trees in a forest.
At this point, Sarasa tells Akiya about how the fish remind her of various Kouka roles. "Those little shiny fish are the ensemble, and those big slow fish are the stars with leading roles." There are so many schools of various big and small fish with such realistic movements that it may be possible that the fish are rotoscoped or filtered footage. Either way, the result is beautiful, and the consistency of their lighting and color as they move around the tank is impressively realized.
Akiya asks which fish represents the top star - for all his flaws, he does care about Sarasa's work. Sarasa says that it is "Definitely the stingray. Its big fluttering wings are like the feathers the top stars wear." The stingray is clearly traditional hand-drawn animation, and its fins' movements which Sarasa describes have a great sense of slow but fluid movement. The way the light of the tank shines more strongly through its thin fins is also an impressive detail.
Sarasa says, "Two years after I start at Kouka, I'll be one of those little fish." At this moment, one such fish is reflected in her eye as it swims past. Not only is this another beautiful detail, but it symbolizes how Sarasa identifies with the fish: it's a reflection, like looking in a mirror. Sarasa tells Akiya that she is finally ready to move on from Kabuki and that she will "let" him be Sukeroku. Akiya is quietly dismayed, perhaps because he believes that Sarasa is right that she would handily surpass him at the role if she were allowed to.
Kageki Shojo!!'s version of Tybalt is, perhaps surprisingly to audiences unfamiliar with Takarazuka, universally assumed to be jealously in love with his cousin Juliet. In Episode 13, Sarasa remembers the aquarium again, drawing on personal experience to strengthen her audition. A tearful Akiya asserts that he will be Sukeroku. This is immediately followed by Sarasa's Tybalt grimly saying, "My life ends by the hand of Romeo," and fading away. Perhaps Sarasa feels that her Kabuki aspirations were finally ended by Akiya's resolve to get the role.
What initially prompts Sarasa to use her frustration with Akiya for motivation is an interview in which he claims that "There were plenty of times I didn't want to be there" as a kid. Perhaps her memory of him finally beginning to actually want the role gave her some relief as well. After all, Sarasa's performance is successful because of how she gets in touch with the character's emotions. Perhaps one of them is forgiveness.
In a funny but important scene, instructor Ohgi explains to fellow Tybalt auditionee Sawa Sugimoto that Sarasa's Tybalt was superior because Kouka fans who identify with Juliet will appreciate how "He only shows his true feelings to Juliet" as he dies. If Sarasa uses her justified envy of Akiya to learn Tybalt's envy of Romeo, and her sense of loss and desire for Kabuki for his feelings for Juliet, then Sarasa too surprises the audience with her true feelings in this scene. The positive, upbeat character, who encourages those around her such as Ai Narata and the Sawada twins, has a secret fury that she shares only near the end of the series. The anime's aquarium flashback reveals what Sarasa must have been thinking as she watched the fish, making her emotions more relatable.