Balala Xiao Moxian: Mofa Hai Ying Bao

In the magical underwater world, there is a mysterious and charming place called Hai Ying Bao, a.k.a. the Ocean Castle. When human girls Ling Haixin and Xia Fan accidentally met the Princess of the Icy Blue City, Na Xiya, they transform from ordinary girls into magic fairies. Together with Xiya, they protect the human world and the sea world, helping her investigate who is polluting the ocean.
(Source: Balala the Fairies Wiki, edited)