Stuff with an * beside it I just made up on the spot so less confident in those sections
Mer AU I've had for like 2 years now
Mer are still a pretty new discovery to the world. Sun and Moon were both raised in a "bad" facility where they were tested and experimented on. Sun ends up losing his eyesight completely in one eye and near sighted in the other. Sun gets rescued and taken into a "good" facility where they decide he can't be released back out due to his inability to take care of himself in general. He makes friends with a caretaker (not the reader) and all is well.
Moon however ends up escaping the facility, it's dramatic and chaotic but he manages to get to the sea. As stubborn and determined as he is, all it takes is fighting around with a shark and a nasty bite to his tail for him to end up with a bad infection. Try as he might he ends up sick and beached. This is where reader finds him.
Reader works at a research center that helps rehabilitate ocean animals. Thinking maybe shark attack or something and reader ends up as a below the knee amputee in one leg. Reader has a friend at the job who's basically a fish doctor/surgeon. They both find Moon at the beach and immediately take him in (after several attempts on reader's life from Moon, he's barely aware but he knows a human when he sees one). Unfortunately because of how deep and far it's spread they have no choice but to remove a portion of the end of his tail. Obviously he freaks out about this, both while drowsy from medication and once he's more aware. Lots of defensive Moon shenanigans, reader works on helping him out and trying to help him understand the situation. They also find a tracker from the "bad" facility (it's been on the news at this point) and now they know he didn't even grow up out in the ocean. This leads to them deciding it'd be best to let Moon have a permanent home at the center. Moon, again, is not happy with this predicament.
A month or two pass and reader and Moon get pretty well acquainted. Didn't really decide if it'd be romantic or not, Moon can speak English but it takes him a while to feel comfortable doing so. Open to interpretation really.
And then there's news of an ill mer at a different center. Sun's caretaker had been becoming more and more sick over the last couple of months. When he was told she wouldn't be coming back, he could put the pieces together, he was absolutely heartbroken. Most mer ends up properly rehabilitated and sent out to live freely. Both facilties are small that each only have Sun and Moon. It's decided that it might help Sun by letting him bond with another mer, he's always been helpful with other rehabilitated mer after all. Also, too much of his home reminded him of his old caretaker. The change in environment could help.
*So. First meeting goes about as well as expected. Despite his recent mood, Sun is pretty excited to meet Moon. An important detail is however left out to Moon. He's aggressive, he flashes his fins, he's learned all sorts of ways to show off aggression and he uses them. But when Sun is still happy go lucky, Moon is simply confused. Even more so when he realizes Sun doesn't follow him with his eyes, circling counts as threatening but its not working. Only when Sun practically shoves his face into the other's and compliments his white freckles does Moon realize.
*After which he immediately shuns Sun. He still sees his slow, awkward swimming and lack of tail fin as a sign of weakness. And now they're throwing in a blind mer to befriend him??? Do they think they'll bond over the lesser parts of themselves??? That way of thinking doesn't last very long though. Sun's too nice, too sweet, too persistent. He can't lash out at him without feeling guilty.
So friends it is. Sun warms up to reader as well. Probably some shenanigan chapters and then they all live happily ever after. Sun x Moon x Reader?? Sun and Moon as brothers and reader in their circle of comfort?? Dunno, either works really
I do really like the idea of Moon and reader both letting Sun get super close to look at them. He has nothing to say but compliments, Moon definitely gets flustered more so. *Thinking Moon has a bunch of scars from tests but Sun doesn't? Again, dunno