The Aquarium

The Aquarium is a seemingly normal location that is located somewhere in town, which Minamoto Kou and Mitsuba Sousuke went to as part of their night out together.

However it was revealed to be a place that housed numerous aquatic supernaturals that roam around the place, as well as creating an illusion of it being filled with people and the sea creatures being docile. The purpose of The Aquarium was to lure human victims inside and then lets the supernaturals within eat them.

After Kou and Mitsuba dealt with the fish-like supernaturals inside and escaped, The Aquarium revealed its true form, which was a gigantic fish-like supernatural that eventually swam back underground.

History
Mitsuba Sousuke and Minamoto Kou visited the aquarium in Chapter 98, as part of their Night Out together. When they came in, it was filled with all kinds of people, and they saw all kinds of fishes and sea creatures. They were amazed by how they looked, and even had fun with each other as they passed through each of them, and they were especially mesmerized by a gigantic ray creature when it passed by the both of them.

After seeing all the different fishes and sea creatures, they went to a marine life keychain automaton, which had five different keychain options, out of which Mitsuba got an isopod and Kou got a penguin, but they decided to switch.

It was after that point that they came into trouble with the supernaturals within. From that point, all that Kou and Mitsuba saw was an elaborate illusion caused by The Aquarium. The people disappeared, having been revealed to be victims of the Aquarium, and all the fishes and sea creatures transformed into supernaturals, especially one that became a large anglerfish-like creature.

Both Kou and Mitsuba and ran away from them as fast as they could, all the while Mitsuba began to lose pieces of himself. They eventually finding a small crevice in an Employee's Pathway to hide.

After they discussed their problems and situation as revealed in Chapter 99, in Chapter 100, both Kou and Mitsuba decided to hold a competition wherein whoever kills the most supernaturals wins and gets to do what they want. For Kou, it's to keep Mitsuba around, and for Mitsuba, it's to have Kou exorcise him. From there, they emerged out of their hiding place and fought the numerous fish supernaturals, with the both of them reaching an even number of kills.

Then they saw the gigantic ray from before, which is revealed to a gigantic multi-eyed ray-like supernatural, and Mitsuba decides that it would be their Tiebreaker. Kou agrees, and the both of them proceed to fight the gigantic supernatural in tandem. After a while, Mitsuba incapacitates the supernatural, and declares his victory to Kou, only for it to reveal its wide, gaping mouth, and attempts to eat Mitsuba in one gulp.

Kou reacts fast and uses all of his exorcist power to fully vaporize the supernatural, saving Mitsuba in the process. From there, Kou deems himself the "Winner" as he was the one that did the killing blow to the gigantic supernatual that was their "Tiebreaker".

When they both got out of The Aquarium, it revealed its true form, roared aloud and sink into the ground. Both Kou and Mitsuba decided that the results of their contest was inconclusive, as the Aquarium itself was the true "Biggest Fish" after all, and left the empty premises together while discussing about the upcoming School Festival.

Appearance
From the outside and inside, The Aquarium seemed completely normal, even having lots of other people inside. It was very large, with multiple exhibits of different sea life creatures. This included sharks, seals, stingrays, worms, penguins and lots of different types of fish. It also has a rest area with food and drink dispensers and a marine life keychain automaton which has keychains.

When the aquarium turned out to be full of supernaturals, Kou and Mitsuba hid in an employee pathway and discussed among themselves on what to do. When they finally decided to escape the Aquarium, they fought the numerous fish supernaturals at the Food Court area, which was filled with chairs and tables.

Creatures
True to its name, the Aquarium hosts a number of aquatic supernaturals of various types and sizes. They range from numerous single-eyed anglerfish-like creatures, to larger, multi-eyed ray-like creatures that have gigantic gaping mouths.

Anglerfish-like Supernaturals
When the Aquarium first revealed its true nature to Kou and Mitsuba, a swarm of one-eyed anglerfish-like supernaturals emerged and began to track down and attempt to kill Kou and Mitsuba. One of them appeared to be larger than the others, and had a darker skin complexion, and longer, sharper teeth than the smaller creatures. The larger anglerfish-like supernatural was not encountered again, and presumably remains alive in The Aquarium.

In Chapter 99, Kou killed one of the smaller anglerfish-like supernaturals in order to take out its heart and feed it to Mitsuba. After an argument between the both of them, Mitsuba finally relented and ate the Supernatural's heart, restoring his strength and reuniting all the broken pieces back to himself.

Later on in Chapter 100, both Kou and Mitsuba killed an large number of the anglerfish-like supernaturals as part of a bet they made against each other. Both of them killed the same number of such supernaturals, making their score even, and forcing them to find a tiebreaker.

Gigantic Ray-like Supernatural
Another type of creature in the Aquarium was the gigantic ray-like supernatural that was dubbed by Mitsuba as "The Winner" for being the Biggest Fish in the Aquarium. It looked like a gigantic Roundel Skate Ray, and had a rather short tail for a ray of its type. In Chapter 98, It initially appeared to be docile and passed by both Kou and Mitsuba, mesmerizing them in the process.

Later on in Chapter 100, after having revealed its true supernatural form, Mitsuba deems it "The Tiebreaker" to break their tied score after having dealt with the smaller anglerfish-like supernaturals and both getting an even score.

In its supernatural form, its size and appearance remain almost unchanged, except that it now possesses multiple large eyes on its dorsal side, and a single large eye on its ventral side. Its tail also grew much longer and now possesses several barbed spikes. The ray supernatural used its tail in an attempt to strike down Mitsuba after he got on top of its head, before successfully striking the soles of his feet and dragging him with it.

After it was taken down by Mitsuba, it opened its gigantic gaping mouth in an attempt to consume Mitsuba whole, before Kou acted quickly and instantly vaporized it with all his power. Its gaping mouth is not only gigantic in size, it's filled to the brim with sharp teeth, deteriorated faces and arms with hands, presumably from human victims lured by The Aquarium.

The Aquarium Itself
The Aquarium in its true fish form resembles a gigantic anglerfish. In Chapter 100, right after revealing its true form, it swam back underground in retreat after failing to kill Kou and Mitsuba with the supernaturals within. When swimming back into the ground, it easily did so, as if the ground was water.

It was deemed by both Kou and Mitsuba to be truly the "Biggest Fish" after all.

Others
The Aquarium is also populated with the numerous souls of all its victims, whose deaths at the hands of The Aquarium's supernaturals was glimpsed by Kou and Mitsuba, as the fish supernatural emerged to kill them.

Trivia

 * Despite Kou identifying the gigantic ray-like supernatural as a "Manta Ray", it does not resemble a real-life Manta Ray at all. Instead, it resembles a gigantic Roundel Skate.