How would you describe this fin better for my book ?

A character's secret daughter from his private family has a tail deformity from a rogue shark attack, and I'm having trouble describing it better

Here's what I have: 

Rio spotted her baby's father, about to go over to the light-skinned African American she stopped when she saw a magenta tailed mermaid with black hair carrying a six-year-old mer-girl whom Rio thought was too old to be held until the mermaid sat the young girl down and the blonde saw the young girl only had one tailfin