The Chaser

Eom Joong-ho is a dishonest ex-detective turned pimp who is in financial trouble because two of his girls have gone missing. One night he sends Mi-jin, one of his few remaining girls, to a customer. He then realizes that this is the same person who was the last to see his missing girls. Believing that this man is reselling his women, he goes to look for Mi-jin, contacting his old police task force for help. But they cannot assist because the mayor of Seoul, whom they are guarding, has just been attacked. The police are now preoccupied with a media storm because they have suffered humiliation for failing to protect the mayor.
Mi-jin awakes tied up in a bathroom. Her "customer", Je Yeong-min, calmly informs her that a previous victim had her tongue cut out for screaming too much. He tries to torture her, but she resists and Je Yeong-min hurts his hand. Moments later, callers from the local church arrive, inquiring about its owner, Mr. Park. Yeong-min has no time to finish off Mi-jin so he invites the elderly couple in and butchers them.
While trying to ditch the couple's car, he collides with Joong-ho's vehicle. Joong-ho senses that the man is hiding something, and calls the customer's cellphone, establishing that this is the man he is looking for. Yeong-min tries to run away but is caught and badly beaten by Joong-ho. Both men are arrested by a suspicious local cop. At the station, Yeong-min casually admits that he has killed up to nine people. A fracas starts when competing divisions all want to investigate the high-profile unsolved murders in the area.
Despite his confession, the police have no physical evidence, so they cannot detain Yeong-min. To get evidence, Joong-ho goes to Mi-jin's apartment to collect DNA samples. In the apartment, he discovers Eun-ji, Mi-jin's daughter, home alone. Reluctantly he takes her with him while he follows up a lead in Yeong-min's home town. There, he learns the suspect had been sent to prison for three years for doing a lobotomy on his own nephew. Joong-ho finds a shabby room where Yeong-min had once lived. On the walls are a host of religious drawings. Eun-ji wanders off, following a woman who looks like her mother. Off camera, she is apparently knocked down in a hit and run. After a frantic search, Joong-ho finds her and takes her to a hospital. He signs the forms as her father.
Time has run out for the police. The prosecutor's office order that Yeong-min be released because the police have no evidence. The prosecutor tells the chief that the arrest, the suspect's injuries and rapid confession will be portrayed as the police's attempt to save face. To avoid political fallout, Yeong-min is released without charge. At the same time, the chief orders the arrest of Joong-ho as a scapegoat because he attacked the suspect. The former cop escapes from the police to continue looking for Mi-jin.
Meanwhile, Mi-jin has freed herself and escaped from the house and the garden full of human remains. Badly injured, she finds help at a nearby corner shop. Yeong-min stops at the same shop to buy cigarettes. The shopkeeper innocently tells him what has happened to Mi-jin and that there is a "maniac" loose. He kills the shopowner before killing and decapitating Mi-jin. Arriving shortly thereafter, Joong-ho finds the street cordoned off by the police and the store a bloodbath.
The police, now realizing that they have again been humiliated, throw everything into the search for Yeong-min. Joong-ho, distraught at what has happened, visits the local church, a link between the house callers and the car that Yeong-min had been driving. Joong-ho notices that the statue of the crucified Jesus matches the drawings he had seen in the room. Inquiries with the deacon lead him to the sculpture "assistant" who was staying at Mr Park's house.
On arrival, Joong-ho discovers a now smartly-dressed Yeong-min, carrying his bag of tools, just about to depart. A massive fight ensues in which a fish tank containing Mi-jin's head and body parts is smashed. Eventually Joong-ho prevails but just as he is about to bring a hammer down on Yeong-min's skull the police burst in and restrain the former policeman. As Joong-ho is pushed face down to the ground, he looks into the dead eyes of Mi-jin.
The film ends with Joong-ho sitting silently in the hospital room alongside Eun-ji's bed. He takes her hand in his own.

Question: What is in the fish that is smashed during the fight between Joong-ho and Yeong-min?
Answer: Mi-jin's head