Help me find old movie, there was an old animated kids movie that started in real life, it was a little boy who left a party at this big castle and goes down to the river on the grounds and ends up falling in and then becomes animated fish and goes on advture with a friend fish... what movie is this

Could you give us any more information about this film? Like how old it is, or any more of the plot? Is it an American film?

Thanks

I believe the movie you're looking for is called the Water Babies.