To quote the summary:
"Gaito has been vanquished, and Mikeru has returned to the ancients. All is peaceful once more for Luchia Nanami and the Mermaid Princesses. But when a new threat strikes back from somewhere unknown, the mermaids must take up their mics once more. This time, though, they won't be alone. Can they succeed like before while keeping their love lives intact?"
Mermaid Melody Pichi Pichi Duo contains examples of the following tropes:
- Adaptational Badass: Over the course of the fic, Kaito, Nagisa, and Masahiro get their own personal idol forms, allowing them to fight alongside the Mermaid Princesses.
- Air-Vent Passageway: In chapter 8, the group uses one to sneak into the aquarium and find the missing Hanon. It ends up backfiring on them when they end up falling down a chute and landing heavily.
- Ascended Extra: Meru only had two appearances in the original anime. She has an expanded role here, even moving into the Pearl Piari. It's the same with Subaru, who only appeared once.
- Back from the Dead: In chapter 3, Dr. Hishki resurrects Lady Bat, Lanhua, and Alala to serve him.
- Badass in Distress: In chapter 12, Lucia, Hanon, and Rina are captured by Dr. Hishki. They are rescued by Kaito, Nagisa, and Masahiro in the next chapter.
- Bedsheet Ladder: In chapter 7, after Nikora declares that the princesses are forbidden to leave the Pearl Piari for any reason, the girls use one to escape through Lucia's bedroom.
- Big Bad: Dr. Hishki, a Mad Scientist who seeks to expose mermaids to the world and make himself famous.
- Birthday Episode: Chapter 11 centers on the group celebrating Lucia's fifteenth birthday.
- Conspicuous Trenchcoat: In chapter 15, Caren, Meru, and Hippo all wear trenchcoats, hats, and sunglasses to spy on Noel. No one, not even Noel, notices anything odd about two girls and a penguin wearing such things.
- Cut Lex Luthor a Check: Dr. Hishki has access to an Underwater Base and a wide variety of Humongous Mecha. He could easily earn a name for himself through other means, but he devotes his time and energy to trying to capture mermaids.
- Glory Hound: Dr. Hishki's only real motivation to out mermaids to the world is purely to make himself famous.
- Implicit Prison: In order to keep them safe from Dr. Hishki, Nikora gathers all seven princesses and declares that they're forbidden to leave the Pearl Piari for any reason until the threat has passed. It doesn't take long for the group to sneak out, and after Nikora has a talk with Maki over it and is nearly captured by Hishki's minions herself, she relents as long as they be careful and watch their backs.
- Love Is a Weakness: In chapter 2, Hippo explains that this is part of the reason he disapproves of the princesses being in relationships with human boys.Hippo: If you mermaid princesses fall in love, you'll start thinking funny. And when you start thinking funny, you might confess you're a mermaid. And when you reveal that you are a mermaid, you'll turn into bubbles!
- Ninja: In chapter 8, at Noel's suggestion, she, Caren, and Coco dress up as ninja to sneak into the aquarium.
- Pain-Powered Leap: In chapter 6, Hippo has this reaction when Taki yanks out two of his feathers in order to tell a fortune, jumping high enough to hit the ceiling.
- Prayer Is a Last Resort:
- In chapter 7, Nikora prays to Aqua Regina for advice, asking what she should do to protect Lucia and the other princesses, or if she can.
- In chapter 15, Caren prays to Aqua Regina as well, wondering whether or not she should embrace her feelings for Subaru or block them out.
- In chapter 20, Meru, out of loneliness, prays to Aqua Regina to help her find love like Lucia and the others have. Her prayers are answered almost immediately; she meets Kaito's cousin Makoto, and it's Love at First Sight.
- So Much for Stealth: In chapter 8, five of the Mermaid Princesses and Nagisa use an Air-Vent Passageway to sneak into the aquarium where Hanon went missing... only to suffer a Prat Fall down a chute and land in a pile.Lucia: So much for sneaking in.
Rina: But at least we are inside now.
- Underwater Kiss: When Nagisa is knocked into an aquarium tank in chapter 8 and nearly drowns, Hanon kisses him to give him air.
- You Can't Go Home Again: With Dr. Hishki hunting them, all seven Mermaid Princesses go on land and hide out in the Pearl Piari away from their kingdoms.