Since they spend a lot of time near or living by coral reefs and brightly-colored plants, their tails are more brightly-colored than others.
Usually a red, pink, orange, or blue tail. They don't have the layer of fat some other mermaids do because they live tropical waters where it stays mostly warm all year.
Just google a pink/red/orange/blue tail, according to which one u want. My internet isn't so good so it's hard to upload pictures in my books. I'll add an aicaya tail inspiration when I can...
I hope this was helpful?