The carp in Yuelongmen refers to the carp living in the Yellow River, which is also explained in Li Yuan's Creeping Things in the Qing Dynasty. What he said was that carp living in the Yellow River wanted to go to Youlong Gate, only to find that there was nothing in front, and there was no water in front. At this time, a carp came up with an idea, saying that everyone should jump hard, and then everyone could see the scenery behind the Longmen.
But everyone is too timid to jump. Then the suggested red carp got up the courage to say jump first. Then he jumped up and jumped into the clouds on the Longmen, and then a blue sky burned his tail. He endured the pain and went down, then went into the water at Longmen and became a dragon. Those little carp were scared when they saw him burned to death. Then a dragon came out and told them that he was the little carp, and the story of carp yue longmen appeared.
Of course, not all carp can cross the dragon gate and become dragons. There are so many carp in Yuelongmen in the story that only one or two of them succeeded. After all, if they had all become dragons, we would surely see them now, so we don't have to see them on TV in textbooks.