Ok, so get this: you're a mermaid(man), any kind, doesn't really matter. And one day, somehow, some sleezebag circus guy snatches you up as a display, and your just fucked. No one knows you're gone, and its not like you can break out and escape since your nowhere near a big enough source of water.
Then, this idiot of a ringmaster decides to go to Metropolis/Bludhaven/Gotham, and ends up getting caught. Unbeknownst to you, you had taken a nap earlier that day and were asleep during all of it.
The Justice league and maybe some of the bats (Nightwing and Signal) just busted this very illegal circus and their going around and freeing all the basically slaves when they happen on you, napping on a rock in your absolutely giant aquarium. Aquaman stands there for a moment, just stunned, and then he's pissed. As you wake up, you're met with the sight of Superman holding him back from going and killing the ringmaster. Once Aquaman realizes you're awake, he rushes up to the glass to reassure you that everything will be ok now.
You're flooded with relief and swim to him and place your hands on his that were laying on the glass, conveying your gratitude in some way (singing, gifting a scale, blowing bubbles). They set you free in the ocean and Aquamna visits you all the time with gifts and stuff, along with whoever else you want.