Carp? I'm Dragon!

Jack works in an office and one day finds himself in another world, in the body of a koi at the bottom of a lake. The mermaid clan that lives there treats him as a joke. Luna pulls him out of trouble, and he ends up bound to a system that lets him evolve by devouring. He works his way up in stages, from koi to a golden flood dragon and then to a larger sea-turning form, beating everyone who comes to challenge him along the way. The Flood Dragon Queen takes notice and accepts him, and for a while he serves under her. Then he learns what she is actually planning. He leads the mermaid clan out toward the open sea, using the power of a nine-tailed bird to get them there, and keeps evolving as he goes, until he stands as a sky dragon strong enough to face her. Ahead of him is the Dragon Gate, which he has to cross to become a true dragon.