Physical change

Ever noticed the chill winter waters no longer bothering you, or the heightened water pressure easing even as you swim deeper? Ever noticed that suddenly you look different, perhaps your eye colour changed or your hair is more curly than it used to be?

Perhaps you have seen other merfolk transforming in front of you. Gaining additional features or changing entirely to look more like their sea creature counterparts.

This is an innate aspect of magic that all merfolk have, though some may be more inclined or prone to use it. This is the most accessible part of magic to most merfolk, and you have probably used it yourself but never noticed.

In principle this magic is easiest to use to transform part of ones body to more closely resemble whatever inheritance flows within your blood.

More talented or well learned can even make themselves resemble humans more, exchanging their tails for legs to blend into human society. Take note that such magics usually have limitations to them in either duration of conditions.

Even more difficult and fleeting is the ability to change their appearance entirely, perhaps gaining wings or other features not innate to either fish or human.

Perhaps you have even heard stories about mysteriously powerful merfolk changing the fates of others by giving legs to those who have not or tails to land bound humans. Perhaps you even fancy finding such and expert and requesting their services.

This one would advise that you empty your head of such thoughts, for unlike one's own magic this one is more permanent and those capable of casting it will inevitably request something in return.