There was a king named Satyavrat. One day, he was doing worship by offering water on the bank of a river. He took some water in his palms. He saw a small fish in that water.
The fish said, "My dear King, why are you throwing me in the water of the river, where there are other aquatics who can kill me? I am very much afraid of them."
The merciful king Satyavrat decided to give protection to the fish. He put the fish in his kamandal(a small round shaped vessel) and brought it to his residence. But in one night that fish grew so much that the kamandal was very small for it.
He put the fish in a large well. Within a moment, the fish became so big that the well was too small for the fish's comfortable stay.
He put the fish in a lake. But the fish became so big that it excedded the extent of the lake water.
He tried one water reservoir after another, each lake bigger than the previous one. But all of them proved to be very small for the fish.
Finally Styavrat put the fish in ocean. The fish said, "In the ocean, there are many powerful and dangerous sharks that will eat me. Therefore you should not throw me in this place."
After hearing these sweet words from the fish, Satyavrat could understand that the fish was none other than Lord Vishnu. He propitiated the matsya(fish) incarnation of Vishnu again and again.
Fish(God) said, "O king, on the seventh day from today, the world will be submerged in water. er of inundation. A large boat sent by me will appear before you. Then you should collect all types of herbs and seeds and load them on that boat. Then, accompanied by the seven rishis and by all kinds of living beings, get aboard that boat. The body luster of the great rishis will show you light. Attach the boat to My horn using great serpent Vasuki. I will pull the boat so long the flood continues. During the journey, you will learn Vedic knowledge from Me."
After saying this, the fish disappeared. After seven days, gigantic clouds pouring water swelled the ocean more and more. Thus the ocean began to overflow onto the land and flood the entire world. Satyavrata saw a boat coming near. He collected herbs and seeds, and, accompanied by saintly rishis and all kinds of beings, he got aboard the boat. A large golden fish appeared. It has a very huge horn.
Following the instructions formerly given by God, the King anchored the boat to the fish's horn, using the serpent Vasuki as a rope. While moving, God, in the form of a fish, explained to Satyavrat the Vedic knowledge.
In the current birth, Satyavrat has been reborn as king Shraddhadev. He is also known as Vaivast because he is the son of Sun god Vivasvan. He has got the post of Manu. He is the seventh Manu.
The story given above happened in the time of sixth Manu, who was known as Chakshus Manu. The fish incarnation also happened at the beginning of the time of first Manu, Swyambhuv Manu. Then God, in the form of fish, killed a demon named Haygriv, who had stolen Vedic knowledge from Brahma and was using this knowledge for wrong purposes.