This was a quite interesting movie. It’s Japanese, from 2011. The main character is a weak-willed fish-shop owner whose life gets entangled with a serial killer’s life. The man is totally helpless against the killer’s strong, domineering personality, and becomes an unwilling accomplice to several murders.
If you like tropical fish there are some pretty specimens in the movie. Also, because I have a twisted sense of humor, a “blood wrestling” scene near the end of the film that I thought was quite funny, although I don’t know if it was supposed to be.