We dropped by the aquarium. From Hongdae, it was roughly an hour commute (Honggik University Station to Samseong Station on the 2 Line). The entrance was bright and had 2 huge cylindrical fish tanks with fish. The admission price was 22,000 won per adult. Arrows on the floor tell you where to walk--the aquarium was basically one road through each of the exhibits.
There fish, but there were also other animals such as rabbits, aligators, etc despite being an aquarium. One of the sections included a hands on lab where you could touch marine animals like starfish or get a doctor fish treatment. Photo points were also scattered throughout the aquarium, but the highlight was definitely the glass tunnel where sharks could literally swim over your head. These tanks contained various types of sharks, sting rays, and sea turtles.
The last stop of the aquarium was the gift shop. There were many stuffed animals, all of which I had to resist bringing home with me, especially the huge shark! I opted for the pressed coins instead, which cost 1,000 won.