Tensei Shitara Kozakana Datta kedo Ryuu ni Nareru Rashii node Ganbarimasu

A guy who has been reincarnated in another world remembers almost nothing about his past life. Contrary to his expectations, he becomes a small fish that wakes up in the middle of the river... A voice in his head, calling itself "the Voice of Heaven," explains the path of his evolution, at the end o