Mermaid Legend (2013)

Besides the title of Mermaid Legend, I've also seen this Chinese show referred to as The Legend of Chasing Fish, Legend of Mermaid and Red Carp Legend among others. Based on the story "Guanyin Yulan Ji" (like 2000 Taiwanese show), this version has Hong Ling (Zhao Liying) as the playful red carp spirit of the title, who has a red CGI mermaid-like tail when underwater. While searching for a suitable present for the Dragon King, Hong Ling pretends to be a human and ends up befriending the provincial and clever scholar Zhang Zhen, who uses his smarts to investigate criminal cases in pursuit of justice.

In this version Zhang Zhen is betrothed to Mu Dan due to a decades-old agreement between their fathers, but Mu Dan has her sights on the crown prince and is uninterested in Zhang Zhen. Worse still, Mu Dan's father had Zhang Zhen's father murdered and is secretly out to sabotage Zhang Zhen's life. This is at odds with Hong Ling's goals, because she needs Zhang Zhen and Mu Dan to get married in order to fulfil a promise she's made to the Dragon King. Hong Ling shapeshifts into Mu Dan's form in order to inspire Zhang Zhen's love for her, but in a fun turn Zhang Zhen recognizes Hong Ling almost immediately, yet because he cannot understand what it means, he decides that he must be in love with both Hong Ling and Mu Dan.

The pair face various trials and solve various cases with the help of their friends over 36 episodes, towards a happy ending of sorts. For about half the show Hong Ling is human, so her mermaid-like tail is seen in the earlier portions of the show and towards the end, and she interacts with various sea creature-based spirits in the show's total-CGI underwater realm. Hong Ling's non-human origins are a complete non-starter for Zhang Zhen when he learns about them, because loves her no matter what she is, but those origins do become a problem in one of the show's plots.

The whole series is on youtube with English subtitles, but you may need a VPN to watch:  Carp Legend playlist 

Thanks to Jay for the head's up about this show.