How to hym a mermaid tail?

[MermaidSolstice]

So my finfun tail wants to live anywhere to just below my bum to my knees. And I'm getting tired of pulling it up every time i surface. I bought the tail based on their size chart but its a little big when I 'm swimming (and its too late to return it).

What is a good way I can take it in or pin it to stay?

I didn't want to use safety pins because it may put a hole in the tail and its not quite big enough to tie a knot in it.

[Mermaid Jaffa]

Easy peasy. Sew in an elastic waistband.

How to measure to swimwear elastic:

Make sure it is swimwear elastic. Other types of elastic will lose its stretch easily.
Do not cut elastic from the roll/yard. Take that length and measure around your waist without stretching.
Cut where one end meets the other end.
Make sure the strip is flat, sew ends together with zigzag stitch.
Here is a video for sewing in elastic into a waistband. They show shorts and skirts but its the same method for swimwear as well. I use the first method in the video.
https://www.youtube.com/watch?v=JJysZyKWx8I