So I made this costume for my niece last year for her Mermaid Birthday party/ Halloween. I used Simplicity 9919. If anyone uses this, the skirt sizing is HUGE! So I'd go down a size. (this is what the pattern looks like http://images.patternreview.com/sewing/patterns/simplicity/9919/9919.jpg
<-- this one is mine
Also, I didn't think the top was very Ariel. So, I made this one by sewing shells onto a ballet leotard to keep her warm and and look more Mermaid like.
That's my niece's dog Ollie. Isn't he adorable?