The main idea of this animation is imagination. It contains a message about being unique, and loneliness and hardships that come with it. This short film is a protest against the exam wars and the mass production of "normal" citizens with no indivisualism or uniqueness.
2. My interpretation
In one scene, the boy shows his parents the mermaid, but they only see a fish. Then they sent him to some sort of facility to get him "fixed". This shows that the boy and his parents might not have had good relationships. It may be the reason why he was looking lonely and depressed at the very beginning of this video. Maybe he wanted someone he could talk to. Loneliness could be the reason why he started to see something in the fish others did not see.
3. About the end
With his power of imagination fading away, he escapes the facility and rushes to his home to see the mermaid, but he could not see what he was able to see before. He then goes to the beach where they first met to release the fish, though surprisingly, the fish once again turns into a mermaid. Afterwards, he turns into a merman and they swim far away from land, leaving everybody else behind.
I think this is a metaphor for if you remember the spirit that you started with, even if your imagination or your dreams begin to fade, there could still be a chance. Moreover, it tells us to do whatever makes us happy and to do that with the right person/people.
