This is what the Agni Purana has to say
In the forest that is known as naimisharanya, Shounaka and the other rishis (sages) were performing a yajna (sacrifice) dedicated to the Lord Vishnu. Suta had also come there, on his way to a pilgrimage.
The sages told Suta, “We have welcomed you. Now describe to us that which makes men all-knowing. Describe to us that which is the most sacred in the whole world”.
Suta replied, “Vishnu is the essence of everything. I went to a hermitage named Vadrika with Shuka, Paila and other sages and met Vyaadeva there. Vyasadeva described to me that which he had learnt from the great sage Vashishtha, Vashishtha having learnt it from the god Agni himself. The Agni Purana is sacred because it tells us about the essence of the brahman (the divine essence). I learnt all this from Vyasadeva and I will now tell you all that I have learnt.”
Do you know what an avatara is? An avatara is an incarnation and means that a god adopts a human form to be born on earth. Why do gods do this. The purpose is to destroy evil on earth and establish righteousness. Vishnu is regarded as the preserver of the universe and it is therefore Vishnu’s incarnations that one encounters most often. Vishnu has already had nine such incarnations and the tenth and final incarnation is due in the future. These ten incarnations of Vishnu are as follows.
(1) Matsya avatara or fish incarnation
(2) Kurma avatara or turtle incarnation
(3) Varaha avatara or boar incarnation
(4) Narasimha avatara- an incarnation in the form of a being who was half-man and half-lion.
(5) Vamana avatara or dwarf incarnation
(10) Kalki-this is the incarnation that is yet to come.
The Agni Purana now describes these ten incarnations.
Agni told Vashishtha the story of the fish incarnation.
Many years ago, the whole world was destroyed. The destruction in fact extended to all the three lokas (Worlds) of bhuloka, bhuvarloka and svarloka. Bhuloka is the earth, svarloka or svarga is heaven and bhuvarloka is a region between the earth and heaven. All there worlds were flooded with water.
Vaivasvata Manu was the son of the sun-god. He had spent ten thousand years in prayers and tapasya (meditation) in the hermitage vadrika. This hermitage was on the banks of the river Kritamala.
Once Manu came to the river to perform his ablutions. He immersed his hands in the water to get some water for his ablutions. When he raised them, he found that there was a small fish swimming in the water in the cup of his hands.
Manu was about to throw the fish back into the water when the fish said, “Don’t throw me back. I am scared of alligators and crocodiles and big fishes. Save me.”
Manu found an earthen pot in which he could keep the fish. But soon the fish became too big for the pot and Manu had to find a larger vessel in which the fish might be kept. But the fish became too big for this vessel as well and Manu had to transfer the fish to a take. But the fish grew and grew and became too large for the lake. So Manu transferred the fish to the ocean. In the ocean, the fish grew until it became gigantic.
By now, Manu’s wonder knew no bounds. He said, “Who are you? You must be the Lord Vishnu, I bow down before you. Tell me, why are you tantalising me in the form of a fish?”
The fish replied, “I have to punish the evil and protect the good. Seven days from now, the ocean will flood the entire world and all beings will be destroyed. But since you have saved me, I will save you. When the world is flooded, a boat will arrive here. Take the saptarshis (seven sages) with that boat. Don’t forget to take the seeds of foodgrains with you. I will arrive and you will then fasten the boat to my horn with a huge snake.”
Saying this, the fish disappeared.