How to hem 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 Momo]

Do you mean the waist is rolling down? To fix that you can add elastic to it

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[MermaidSolstice]

It's not really rolling. It's sliding down to my knees, I got a smaller sized tail for Christmas and it fits fine so far (haven't swam in it yet). I really didn't want to add elastice because, sadly, I am tall, and the tail is almost to short and I don't want to lose any length.

[Mermaid Jaffa]

Sew on extra fabric, then use that extra fabric to make an elastic waist.