Ok so there's this anime i remember seeing back in high school and i can't remember the name of it, But i can remember an episode of this anime. The show was a kind of gory mature rated show, and it was about a young lady who destroyed these supernatural monsters. But here's how the episode goes:
A young male is wandering inside of a aquarium and stumbles upon a closed off section of the aquarium, and discovers that there is a mermaid in the aquarium. A lady who runs the aquarium steps up and tells the guy to leave and forget what he saw, but he refuses to do so. The aquarium director allows him to come back and visit the mermaid, but has to keep it a secret. He comes back a second time to visit the mermaid and somehow manages to bond with it telepathically, seeing the fantasies of the mermaid wanting to go back to it's home in the ocean. The director tells the boy that the mermaid is sick and will be sent back to it's home soon. The boy is disappointed to hear this and decides to come back to visit the mermaid once more, except this time he wanted to get up close to it. He's standing at the edge of the tank that the mermaid is in and fall into it. The guy see's the mermaid and is excited, but it starts to mutate and devours the boy. Then the heroin steps up and breaks the tank, letting out the creature, as it still is mutating into some strange monster. The heroin then fights this creature and defeats it.
Hopefully someone has seen this anime and knows the name of it. Hopefully my description of the episode is clear enough to understand.