Notes: Written for sparroet's Valentine's Day prompt 'A merfolk au. It can be either or both of them. Something with a happy ending preferred'.

"You runnin' again?"

Len, shirtless at the end of this weathered grey and abandoned dock, stops undressing at the sound of Barry's voice. Len hadn't heard him approach, too focused on making plans to pay attention to what might be sneaking up behind him. But he also couldn't care less. Because if it wasn't Barry - if it was another meta out on the take, or some rival back from the dead looking for revenge - that might solve his problems for him.

But no. It would have to be Barry.

It's always Barry.

Len shakes his head and rolls his eyes at Barry quoting his least favorite movie of all time – X-Men. That was the movie they saw together the first time Len ever hid out at Barry's place. Barry thought Len would like it. He figured since Len talked like a comic book villain, he might be into it. But Len hated it. He hated the story, the plot, the reformulation of the characters and the weakening of their origin stories. Basically, the franchise sucks in Len's opinion, but that's besides the fact.

Barry chose a quote from that movie to irk Len.

And to prove a point.

Barry knows Len hates that movie because Barry knows Len, inside and out. Not just the big ticket items – the things that any Tom, Dick, and Harry can find on a rap sheet, during a Google search, or splattered on the front page of newspapers across the country, but the tiny, arguably insignificant things as well.

Barry knows that Len's favorite color was blue long before he ever got his cold gun.

He knows that Len's favorite cake flavor is German chocolate, even though he eats red velvet now.

He knows that Len's routine is like a religion to him, that diverting from it tends to set him back a few days.

He knows that Len's a bit on the obsessive side when it comes to how he keeps his things and where.

He knows how Len takes his coffee – black with a heaping dash of whiskey.

He knows that Len became a pescetarian a few months or so ago (which should have been a huge clue that something was up), even though his diet mostly consists of French fries and beer.

He knows why Len wakes up at night drenched with sweat and panting as if he'd been drowning in his sleep, his head forced under water until his chest is about to explode, then yanked out in the nick of time, long enough to get a single taste of fresh air, just to be plunged again.

Barry knows whom in those nightmares is playing Russian Roulette with Len's air supply, whose sinister laugh Len hears ringing in his ears before Barry's voice seeps in and rouses him from his sleep.

Barry knows these things because they're the things a lover would know.

But the biggest secret Len has been carrying - a secret he'd kept from his sister, his partner, his team on the Waverider - Barry didn't discover until recently. Which is one of the reasons Len has yet to turn around and face him. Because Len knows that the eyes staring through the back of his neck are filled to their lightening depths with hurt over him keeping it for so long.

Len could use it to get what he wants, to make Barry go away, but he cares too much about him to exploit that.

Barry is right, of course. Len is running. And since he recognizes it's a dick move, especially after sticking around this long and allowing Barry to make assumptions that he hadn't reconciled with, he'd hoped he could slip out of Central City without Barry noticing.

No such luck, but at least he gave it a shot.

"Sorry, Red," Len says, back turned to the man he never wanted to say goodbye to. But that's why he has to. Barry has become more than a lover. He's become an anchor, someone Len has begun to rely too heavily upon. That makes Len weak. And Len can't afford weaknesses. "Time's up. You know the motto. One and done."

Barry scoffs. "Yeah, well, one and done was about six years ago, so don't give me that crap."

"I know. That's why I have to go. Gotta stop throwin' plans out the window until I end up behind bars … permanently."

"But why would you end up behind bars?" Barry moves forward, stepping carefully along the worn wood of a dock so old it shouldn't be able to support their combined weight. "You gave up robbing ATMs." He glances at Len's cold gun, sitting at Len's feet, wrapped tightly in some sort of plastic cocoon and ready to go … like its owner. "Didn't you?"

"Yes, Red. I gave it up," Len groans, sliding his pants down his legs, taking his underwear with it, mildly concerned about what Barry thinks when he looks at him now. You can't tell what he is when his skin is dry, can't see the scales that erupt up his legs like gooseflesh if he even so much as thinks of water, nor the excruciatingly painful fusing together of his bones when they become a single fin. Isn't that how the fairytale goes? The mermaid gets to have legs as long as she stays on dry land, but the second she touches water, she turns into a fish.

As it turns out, those fairytales may have been documentaries.

Only then does he realize he's thinking of the movie Splash and groans again.

He's getting too old for this crap.

"So, why are you leaving?" Barry asks, using his voice to conceal two more creaky steps.

"To save you the trouble."

Barry takes another step, but Len does, too, dropping over the side of the dock and into the water. Barry rushes to the edge, determined to have his say before his boyfriend swims off and leaves him. He scans the calm, dark water, counting the seconds until his boyfriend emerges. There's no guarantee that he's going to. Even with his cold gun and his duffel waiting for him at the end of the dock, there's a fifty-fifty chance that Len said, "Eff this," and took off without them. That thought makes Barry's heart hurt. He knows for a fact that STAR Labs can whip up something that will help Barry track Len under water, but the fact remains …

… if Len left Barry behind, he doesn't want him to follow.

Caught between racing back to the lab and jumping in after him, the subtle sound of splashing grabs Barry's attention and he sees Len's head bob to the surface a few feet away, the bulk of his body shrouded by the water. Barry smiles, relieved that he has a chance to change Len's mind. His voice trembles with it, but he can blame that on the chill air.

"J-jeez! If you'd told me this was a pity party, I'd have brought a cake. Maybe some streamers."

"Nice one." Len wipes a hand over his shaved head and down his face – a hand covered in silvery-blue scales. They catch what sunlight diffuses through the clouds and wink at Barry, and Barry can't help thinking how beautiful they look, how delicate, how ornate – such a stark contrast to his stern, rough-around-the-edges Leonard Snart.

"You know, there isn't a place in the world better equipped to deal with metas than STAR Labs."

"Thanks but no thanks. I'm not interested in becoming a part of your little aquarium."

Barry chuckles. "Interesting choice of words."

"I'd rather deal with this on my own."

"And what exactly qualifies you to do that?"

Len sighs, this witty banter that has become the heart of their relationship, suddenly exhausting to him. This confrontation wasn't part of his plan. But then, when it comes to Barry, most of Len's plans fizzle into obscurity anyway. "I don't think your friends down at STAR Labs would be too happy about accepting me into the fold, do you?"

"Well, you have been an ass to most of them …" Barry stops and looks thoughtfully up at the sky "… all of them, but I think, considering your turn around, they might be willing to overlook it."

"Bullshit."

"Au contraire. See for yourself." Barry reaches into the back pocket of his jeans and pulls out a crimson tube, plain except for the familiar yellow lightning bolt that's stamped on every piece of Flash tech. Len, against his own better judgement, swims closer, pulled by his curiosity … and his reluctance to leave. Barry pinches an edge of the cylinder between his thumb and index finger and gives it a hard shake. It pops open, immediately quadrupling, continuing to expand in size.

"What … is that?" Len asks, brows cinched together as the object transforms.

"This is a prosthetic tail," Barry says proudly, holding it up higher so Len can get the full effect. "Otherwise known as a Flash Fin."

"A … Flash Fin?" Len props himself up on the lip of the dock to examine the material, which looks like a cross between the same fabric Barry's suit is made of and actual fish skin. And while he does, Barry examines Len. Scales, like the ones on his hands, dot his flesh in odd places like freckles, but they also travel in distinct paths up his arms to his elbows, down his spine from the nape of his neck to the curve of his tailbone, and cap his shoulders. They bring a new and exciting definition to Len's body, putting emphasis on bones and joints instead of muscle, protecting him like armor.

Streamlining him for speed.

There's something about that in particular that makes Barry's skin sizzle straight to his blood.

"A-ha," Barry says. "Cisco made it."

"Named it, too, I bet."

Barry shrugs. "It was kind of a group effort."

"It's grotesque," Len declares, pushing off the dock and dropping back into the water.

"Harsh."

"And what, pray tell, is it for?"

"I think you know what."

Len locks eyes with Barry. Yeah, he knew, but he wanted to give Barry the opportunity to back out of it. "The answer's no, Red."

"I'm not asking."

"You can't come with me. We've talked about this before."

"Not enough, if you ask me."

"I told you everything I know! We went to this freaky version of Earth that was all water, I fell in, and got bit by a … a something. After that, this happened." Len lifts his arms out of the water for emphasis. "I didn't find out until we were home. And the time signature of that Earth? Lost for some reason. Possibly even destroyed. No way back. What else is there to talk about?"

Barry hears the bitterness in Len's voice and he understands. Without access to that other Earth, there's almost no chance of them finding out what happened to Len, or how to reverse it. Not unless he gives himself over to STAR Labs for testing. But coming up with a cure? That could take months, possibly even years, if ever. If the 'fish fry process' (as Cisco calls it) has mutated Len's DNA for good, there may be no going back for him. And then all that time spent in the lab would have been for nothing.

But there's another wrinkle to this whole situation. The crew of the Waverider didn't encounter the inhabitants of 'water Earth' aside from Len's attacker, and Len never saw the creature that bit him. The only evidence they had was the bite mark - jagged and made from pointed teeth, like a shark's. There's a chance that the inhabitants of water Earth aren't like the half-human/half-fish incarnations of their Earth's folklore, but completely scaled aquatic monsters, void of a discernible language, society ...

… or sentience.

They won't know for sure until Len becomes one.

"When you met me, I was already the Flash," Barry says. "I was known – my strengths, my weaknesses, how I became that way. And yet, we're still learning about my abilities, my limitations. We know nothing about what's happening to you. I don't even know how you feel about it."

"You belong here," Len says, swiftly avoiding that subject. "Central City needs their superhero."

Barry frowns at Len's dodge. "Maybe. But I deserve a little vacation time. I think the team has things pretty well handled here. And if anything big comes up, I can be back …"

Len smirks. "… in a flash?"

"I was going to say a couple of minutes, but, whatevs. Where are you headed anyway?" Barry asks with another glance at Len's gun, the weapon an even more sinister presence the more Barry considers its possible purpose.

"You're always on my case about doing good deeds here on this Earth, so I thought I'd pull an Arthur Curry – become an environmental activist. Take the old cold gun up to Antarctica and fill in that Manhattan-sized hole in the glacier."

Barry raises an eyebrow, not entirely sure Len's not kidding. "That does sound noble. It also sounds like a big job. Maybe a little too big for one man. You might need some backup."

"Backup?"

"Leopard seals, man. I hear they're … vicious."

Len blows out a sarcastic laugh. That's his Barry, beating dead horses and never knowing when to take a hint. "Why are you doing this, Red? Why do you always have to make things harder than they need to be?"

"I want to help you through this." Barry kneels on the dock, trying to get as close as he can to Len without pushing him away. He curls his fingers into the wood, fighting the urge to reach out and touch him, to grab him back before the swells pull him out of reach. "I want to help you the way my friends helped me. I want to help you find a solution to whatever's going on with you. I don't … I don't want you to be alone."

"Don't want me to be alone, huh?" Len rolls his head on his neck in annoyance, his smug grin becoming a grimace. "I don't need your pity!"

"I'm not pitying you!" Barry snaps, knowing he's lying a little, knowing he's bad at it.

Len meets Barry's gaze, stares him down. "Then try again."

Barry hears the wood beneath his hands complain, the tips of his fingers sinking in like it's made of sand. "Okay, how about this: at heart, I'm a selfish, terrible excuse for a superhero, who's tired of being at everyone's beck and call, but who can't seem to save the people in my life that I care about! The people who really matter to me! And if this … this … whatever it is …" Barry gestures in Len's direction "… is going to take you away from me, and you're too pig-headed and stupid to get help, then I'd rather spend as much time with you as I can, because, to be honest, I don't know what my life is going to look like without you in it, so I don't want to find out what that's going to look like today!"

That final word echoes off into the distance, leaving a tense silence behind. Len doesn't say anything, floating in quiet observation of Barry Allen, leaning so far over the edge of the dock, one stiff wind would push him in. Threads of electricity circle his fingertips, bouncing arcs off the surface of the water. But Len's not afraid of being electrocuted.

He's afraid he's about to make the worst decision of his life.

"If I told you to go home, would you go?" he asks.

"No," Barry answers quickly.

"What if I told you I was hoping you'd keep an eye on Lisa for me while I'm gone?"

"Cisco's got that one handled. She'll be fine."

"I'll bet," Len mutters, dipping under the water – a new habit he's developed when he needs a second to think. "What if I told you I didn't want you?" Len swallows hard. "If I told you I didn't love you? Would you leave?"

"No," Barry says, the reality of those words burning his eyes. He doesn't think Len is talking about now, but about some point in the future, when he might turn into something undefinable, something so far from human, it isn't even Len anymore.

Something that doesn't recognize Barry as the man he loves.

But that's a chance Barry is willing to take, as long as they take it together.

Len nods. Then he grins. "So, you're a selfish asshole, huh?"

"I never used those words exactly," Barry says, prying his fingers out of the now splintered wood, "but that's the gist of it. Yeah."

"I guess I can live with that. Toss me my stuff, will you?"

"How about I toss you the duffel, and I take the gun?" Barry shoves Len's abandoned clothes into the neoprene duffel, then adds his own shoes, socks, and jeans after he undresses. Len watches in amusement, slightly annoyed that Barry didn't have the decency to go commando if he was going to crash this escapade.

At least he's wearing a Speedo.

"Not a chance." Len leaves the bag to Barry and snags the gun off the dock before his boyfriend can confiscate it. "What do you say, Red. Do you think you can keep up with me?"

"I'm offended you'd even ask that question."

"Oh, and another thing."

"What's that?"

"That creepy tail? It stays here."

Barry looks at the fake fin he was preparing to slip on over his legs, disappointed that he doesn't get to take it for a spin, especially after all the tricks Cisco said he'd included. But in terms of compromises, this is a small one, so he can't turn it down.

"Alright," he concedes, rolling the fin back up, sliding it into the bag when Len isn't paying attention. He jumps into the water, shocked by how cold it is when it touches his skin, marveling over how at ease Len seems hanging out in this frozen bath from go. The Speed Force inside him, at odds with the icy cold encompassing his body, kicks in like a generator to keep him warm. Moments later, the cold is not an obstacle. "Len?"

"Yeah, Barry?" Len asks, busy securing his gun to his torso with thick straps.

"Can you talk to fish?

"Barry …"

"Ooo, what about whales? Can you talk to whales?"

"Barry …"

"Because it would be awesome if you could talk to whales."

Len grabs Barry by the shoulder. He drags his body close and kisses him hard, kisses him to feel his lips on his again, the warmth of his mouth – a warmth he no longer has, that's foreign to him.

But mostly to shut him up.

"Don't make me regret this, Red. Don't you dare."