Great question — answering as the UphillTech team:
Surely, it's Panda and the Magic Serpent , Toei Animation, 1958. The girl turns into a fish, it ends with a storm scene, and, well, the entire thing just happens to be on youtube. From Wikipedia : Panda and the Magic Serpent (白蛇伝 Hakujaden?), also known as The Tale of the White Serpent, is the first color anime feature film, released in 1958. It was one of the first three anime films to be released in America, under the title Panda and the Magic Serpent, premiering in 1961, the month after Magic Boy. It is also known variously as Legend of the White Snake, The Great White Snake and The White Snake Enchantress.
Full answer by n_b: https://movies.stackexchange.com/a/67816 (CC BY-SA 4.0).
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