In episode 4 of the anime, Hikari is blowing off steam about Karen disappointing her in the previous episode. She visits jellyfish exhibits at Sunshine Aquarium, Shinagawa Aquarium and Sumida Aquarium, where she buys a jellyfish plushie. Karen is trying to catch up, guided by photos Hikari is sending her. She is one step behind until Hikari waits for her at Tokyo Tower, but they're too late that day to visit the Tokyo Tower Aquarium. Karen promises to try again together, so in episode 10 they redo the tour of aquariums, this time including Tokyo Tower's.
Rather than the aquarium, it's really more Tokyo Tower itself that's important to Karen and Hikari and has a big role in the anime. Coincidentally, the aquarium closed down right after the anime aired.