In the late 1920s, Walt Disney started a series of cartoons labeled "Silly Symphonies". These shorts did not feature the usual Disney characters but consisted of various animals or bugs (in most cases) moving about to the music. While the style is pretty antiquated compared to later cartoons, they're pleasant and a lot better than the competition--who started copying Disney with similarly named shorts (such as Merry Melodies and Happy Harmonies).
"Frolicking Fish" is pretty typical of these films in many ways. While it is odd that it features fish, the rest is pure Silly Symphonies. Cutesy creatures dance about and frolic. However, the fun is cut short when an evil creature (in this case an octopus, but in others it's a cat or bear or bird or some other nasty) appears and wants to do harm to the super-cute folks of the sea. Fortunately, the fish it pursues is pretty handy and the day is saved....huzzah! Overall, while not terribly original, it's pleasant and easy to watch.