Answer:
The tale about the fish is a metaphorical story that Dorothea used to talk about Joe and his need to stop his 'tunnel-vision’ and focus on things around him too. Joe thought playing with Dorothea will be 'different' and would make him famous and rich, but that wasn't the case. He was still the same Joe, riding the same subway, leading the same life. In fact, nothing ever changed, which confused him.
Step-by-step explanation:
The Pixar movie "Soul" is not only a beautiful animated movie but also a deeply thought-provoking movie dealing with existentialism and the importance of one's life aims.
In the movie, Joe Gardner was and his only goal was to play alongside the famous jazz artist Dorothea Williams. And in this desire, he forgot to think of his other relationships, be it with friends or families. His sight was just a one-way tunnel, with Dorothea at the end of the tunnel.
When Dorothea told him about the fish, she is using a metaphorical tale to tell Joe about how it is important to be aware of the surroundings. Joe had believed that playing with Dorothea will be 'different, but when he actually played for her and he felt nothing different or 'great', he was confused. He did not even know how or what to do next after he had 'achieved' his goal. So, Dorothea's story is about Joe and his tunnel vision that blinded him to other things around him.