Board Thread:General Discussion/@comment-25850402-20150723062747/@comment-25077005-20150822043138

Eskaver wrote: Squid ink doesn't effect the Mermaid's transformation at all, that's to freeze powerful magic users.

As we know, Ariel cannot change her tail at will unless she has a transforming bracelet. Mermaids typically do so via the (real) Goddess Ursula. It's not somedifferent type of magic just the same shapeshifting magic as always. Regina managed to turn herself into a cecaelia with her normal magic (just like when Ursula used the trident and what the Goddess Ursula already is.).

We don't know that Poseidon's trident has anything to do with mermaid tail transformation either.

SO, confirmed ways are only, Goddess Ursula and Enchanted bracelet. Turning into a cecaelia is trident and general witchcraft. About the transform tail at will. While full mermaids can't there could be a slight chance a mermaid born half human half mermaid could transform at will.