Raidorl Seiken Senki - Chapter 11: Chains and Witches.

A story of Gin, a young man who lost his memories before the age of 17 and became a magician. Living a quiet life running a cafe while researching magic, he is suddenly thrust into an unknown world. Moving between Earth and a realm filled with danger and secrets, Gin must rely on his knowledge, skills, and magical prowess to navigate the perils that arise. Ruthless and determined, he will stop at nothing to uncover the truth behind his lost memories and the mysteries of the world, using every tool at his disposal to rise above all

However, he did not know that he was not raising just a harmless pet, but a biological weapon secretly made by the government… a real monster.

As a former librarian, Rand felt that he had been living an ordinary life, the one that he had reclaimed unconsciously every single day.

Recently, he has been a little upset because the pet fish he accidently brought home to raised seems to have undergo some strange changes

Well, it turned into a merman.

- Description from Novelupdates

200 years after the world has ended, the laws of society have long been forgotten.