Ningyo
English name: Mermaid
Genres: Fantasy
Type: Movie, Episodes: 1
Aired: Sep 21, 1964
Studio: Mushi Production
Source: Unknown
Rating: G - All Ages
Synopsis:
In a fictional place where using the imagination is banned, a boy saves a fish, which surprises everyone by turning into a mermaid. The boy is arrested for imagining this "nonsense", and is robbed of his imagination as punishment. 

(Source: ANN)