It's so strange that people with reading comprehension that never passed sixth grade have so much disdain for this book. I'm sure that if you wanted to do a close-reading analysis of each and every component, you'd find issues, but at its core, this story is about being born with privilege and using that privilege to support others who weren't born with that privilege--it's not even like his parents worked harder to give him an easier life or something, he just happened to get lucky (not terribly unlike someone being born white in America). The Rainbow Fish and his family didn't work any harder or do anything better than the other fish, he was simply born that way. Maybe there's something to criticize in him giving up parts of his body, but it really doesn't sound like this caused him any pain or harmed him in any way beyond the loss of something that made him pretty. The scales in this children's story are cosmetic and fairly superficial. I strongly suspect that the people who have so much hate for the "socialism" and "communism" in this book also take issue with wearing masks because it somehow restricts their freedom and they can't be bothered to endure mild discomfort in order to protect strangers.
Anyway, I'm in university now and I haven't read the book in years, but keep a poster of the cover image on my wall as a reminder to strive to be my best, most altruistic self.