I found the pattern for this blanket https://yarnutopia.com/2015/04/29/how-to-crochet-video-tutorial-mermaid-tail-afghan-pattern-3-sizes-small-child-medium-teen-large-adult/
The pattern called for changing yarn colors every row, but I found this beautiful yarn with shades of blue and green in it and thought that it was perfect for a mermaid tail, so I decided to go with that instead of changing yarns. The yarn is a color called Pond from the Deborah Norville Everyday yarn line from Premier Yarns.
Here is a close up of the shell stitch. I love the way the colors of this yarn transition with this stitch.
The pattern for this blanket has directions for different sizes, but basically each size is adjusting for the length you want the blanket to be. I did alter the pattern slightly for mine towards the bottom of the blanket . . . when it started to decrease the stitches to close up the bottom of the blanket. I alternated a row that decreased with a regular stitched row so that it was a slower decrease and slightly longer blanket than what the pattern called for.