It's a manga about schoolgirl mermaids. It's not that great, to be honest, but it's relaxing and its mermaids are interesting.
Firstly, they're not particularly mermaid-ish. They go to human school on dry land, by taking a medicine that temporarily turns their fish tails into human legs. If they forget to take it, they go fishy again. This sounds more like magic than medicine to me, but the whole thing's light-hearted fantasy anyway, so what the hell. Their human schoolgirl forms are almost completely human, by the way, but they'll have fins growing from their necks, a mouth full of jagged shark teeth or the like.
Secondly and more interestingly, they're not generic mermaids. They're all very specific fish and their tails are all kinds of weird shapes, almost none of which will be the standard elegant shape. This show's cast includes:
1. Wakasa Otomi, token human and mermaid lesbian fangirl. She practically drools over her piscine classmates, no matter how freakish or ugly, and is capable of getting nosebleeds if she sees their fish tails. She shares mermaid pin-up magazines with the boys in the class.
2. Tsutsumi Anko, an Atlantic Footballfish and one of the ugliest things in the sea. These are deep sea angler fish, so Anko has a fishing rod growing out of her forehead and a glowing ball on the end of it. As a human she looks like a cute manga schoolgirl, like all these mermaids, but her fish tail is a blobby, freaky-looking mutant horror.
Being a deep sea fish, Anko is gloomy and unsociable.
3. Suzuki Touna, a Japanese sea bass or fighting fish. The real thing looks beautiful, but Touna is a bitchy show-off and not particularly bright. As someone says to her, "Why don't you go attack your own reflection?" (Fighting fish will do this. Weird quirks or biological peculiarities that a fish might have in real life will always be reflected in their fictional equivalents here. That's the most interesting thing about this manga. It's educational.)
4. Fukuda, the delusional teenage Red-eyed Pufferfish. Not only can she swell up to look like a Sumo wrestler (which looks funny), but she could kill anyone she touched with deadly neurotoxin.
5. Shiori, a dangerously fast Pacific Needle-Fish mermaid who'll come at you fast enough to knock holes in the wall. Apparently they're one of the most dangerous fish in the sea, known to stab and kill divers simply because they're fascinated by lights. Bizarrely, the people Shiori targets are people she finds attractive.
6. An eel mermaid! "When I'm with a person I l-like, my entire body becomes slimy and sticky with mucus..."
7. The Ocean Sunfish Mermaid. Her idea of sunbathing is to be attacked by sea birds, while she's capable of asking her classmates how many eggs they've produced lately. "Yesterday was crazy for me. Fish like me can produce up to three hundred million at a time."
All this is wacky. Fish are weird. Schoolgirls that behave like fish are even weirder, especially when the author is putting scientific names and explanations at the bottoms of panels as footnotes. Annoy Anko and she'll flash at you, which is less fun than it sounds. (That headlight of hers can temporarily blind people.)
There's lots of schoolgirl lesbianism, but no actual sex or romance. There's nothing serious about this story. It's all for fun. The art's also completely wholesome, with no real fanservice and the style in general being simple and generic. The schoolgirls are cute. Not sexy or anything like that, but cute in the style of a children's series. If you're looking for a non-exploitative monster girl manga, read this. It's a pleasant, relaxing series... but at the same time, it's not actually that good. Inuinu is a first-time mangaka, according to the end of vol.1 where he says that when he started writing and drawing this series, he didn't know right from left. There's thus nothing unremarkable in the storylines. There might be conclusions that don't quite feel earned (e.g. ch.2) or business that in general just feels a bit formulaic (e.g. ch.11).
I like it. I enjoyed it. It's nice. However what I was enjoying was basically a cast of eccentrics bouncing off each other in a comfortable story setting. The manga doesn't really seem to be aiming for a story. Incidentally, I think I saw online that the series is now complete at only four volumes, but I can't find the reference any more and I might be wrong.
Would I recommend this manga? Maybe I would, yes, if all you wanted was something undemanding and pleasant. The detailed fish research is cool. We have gross weird fish tails. Wakaba's complete and unconditional acceptance of everyone is rather sweet, even if it's obviously based on fancying the arse off all of them. No matter how bizarre, dangerous or ugly someone's hidden fish identity might be, Wakaba will bliss out over it. I liked it.
"Finding a breeding partner is difficult in the deep sea. In some species of the footballfish family (Himantolophidae), the male becomes parasitic with the female in order to miss no opportunities. The male fuses, then loses its internal organs except for its gonads, which continue to develop, to concentrate resources on producing offspring."