Such as how they said that if you carry a mermaid's hair or scale, then you will carry the mermaid's pheromone. However, when Bai Jian give Si Yue a bracelet woven with his hair, there was no mentioned about how people can smell the pheromone. Moreover, the bracelet was never mentioned ever again. How could it suddenly became invisible when it is said to be such a precious thing as it was woven by Bai Jian's himself and contains his hair?

Also when they said that they will followed Bailu's wish to get treatment because that's what he wanted. They can't decided anything because it is what he wanted. But then after Si yue successful transformation into a mermaid, they just decided between themselve that it is time for him to leave home with Bai Yueye to live his life? Without asking him if he want to go out ang enjoying the life he never had before he died, or spend the remaining times with people he loved.

Another thing is how it was said that mermaid is way stronger than human. Then why did when they go to the 3rd research institute, the mermaids researcher there is not as strong as Si Yue who just turned into mermaid? I know he got a strong mermaid gene, but what baffled me is how the researchers (who are a mermaid) become as weak as human? When they supposed to be way more strong than human?