I read this book in the eighties but am not sure of the publication date. A girl must save her baby brother who has been taken by some kind of moon-witch and turned into a silver fish. If she doesn’t save him before the next full moon, he can never be changed back. She has to make a potion which includes an icicle from the moon witches’ cave (and maybe some tears?). I think she gets help from some animals, possibly a fox.
This sounds like ‘The Stream that Stood Still’ by Beverley Nichols. This is the second book in Nichols’ Magic Woodland Trilogy. The first book in the series is ‘The Tree That Sat Down’ and the third book is ‘The Mountain of Magic.’
When Jill’s brother, Jack, is kidnapped by the wicked witch, Mrs. Smith, and turned into a fish, it is up to Jill to save him – before he becomes trapped as a fish forever. Jill is able to survive underwater with the help of a breathing mask made from water lilies by her grandmother (or mother?)