I like mermaids. No, actually, I REALLY like mermaids. Whenever there's something even remotely mermaid-like, I jump the wagon to read/watch/own it.
... and, well, sometimes that leads me to this kind of stuff.
Momoiro Ningyo is about a mermaid (Run) who falls in love with a human (yeah, like we've never seen THAT before) and decides to grow legs so she can be with him. And by "be with him" I actually mean: HAVE SEX WITH HIM.
Problem #1: When she gets horny, her legs instantly turn into a tail, so... even if the guy wanted to, he CAN'T have sex with her
Problem #2: the potion she
took to become human has an "expiration date", and if she doesn't have sex until then, she'll become a merman just like her eldest sister.
And that's about it for the plot. No more, no less. And what's worse: these problems were only revealed to us by the end of the first volume! So, before that, it was just this cute mermaid trying to get laid with this guy who's clearly not interested.
I forgot his name, but I really don't like him. He's always angry with her, often thinks of her as "annoying", and the only reason why he accepts having sex with her (only to find out that he can't) is because he doesn't want her to become a man. Yep, transphobic much?
I was going to give this story another shot but to be quite honest, I don't think it gets "better" than this.