Don't know if it's been here, but here's an awesome female to mermaid tf. The movie called "Blue my mind", it's swiss, so it's entirely in swiss/german. The tf itself is slow, it goes over the course of the movie starting from tiny webbings between the girl's toes and ending with a long mermaid tail (gills included). 100% worth checking out
Here's the cut of most of tf scenes
https://www.youtube.com/watch?v=gIUswcGbwTM