Today is Matsya Avatar Jayanti and hence this is about Matsya Avatar of Lord Vishnu.
Matsyavatar - The Fish incarnation. This is The FIRST AVATARA of Lord Vishnu in Krita Yuga.
At the end of one of the Kalpas, after a day's strenuous work, as nightfall was approaching Brahma felt sleepy. As his eyelids closed and his mouth opened involuntarily for a yawn the Vedas slipped out of his mouth without his noticing. But a demon by name Hayagriva did not fail to notice this and hurried to devour the Vedas. Sri Vishnu who is all pervading and omniscient observed this and decided to retrieve the Vedas and restore them to Brahma to enable him to go ahead with his work of creation during the next Kalpa (day break).
While Sri Vishnu could have wrested the Vedas from the demon by his sheer will-power (Sankalpa), he chose to wait for performing a two-in-one feat of conferring his blessings on a Bhakta, SATYAVRATA by name while destroying the demon. As the deluge (Pralaya) was near at hand, he decided to descend `down to earth' in the form of a `fish' to accomplish both the tasks.
Satya Vrata was a great and good King. Once he went to the river KRITAMALA for offering water (Argya) during Sandya vandana. A small fish came into his hands as he cupped his palms to lift water. He dropped it back in the water. But every time he lifted water the fish was sure to get into his hands. The King left it in a tub but it grew so fast that the tub could not hold it. The king transferred into a well, pond, lake, river and finally the sea as it overgrew so rapidly that none of them could contain it. Satyavrata was amazed, when the fish revealed himself as Lord VISHNU. When the Lord asked the king what he wanted, he did not desire anything for himself. He did not even want Moksha. All that he sought was that even during the impending Pralaya; he should be instrumental in saving the life of worthy souls from destruction.
The fish told him that on the 7th day from then a great deluge (Mahapralaya) would take place when a severe tornado would rip through the Universe and destroy it. But, if Satyavrata could muster herbs, seeds and a pick of beings he wanted for the next Kalpa and keep ready, the fish would send a spacious boat in which all of them as also the Saptarishis (7 sages) would be accommodated and saved. The fish advised that Vasuki, the serpent should be brought and used as a rope for fastening the boat to the horns of the fish. Satyavrata did exactly as advised by the fish and the entire crew was saved. As the boat sailed throughout the night of Brahma, Lord Vishnu taught Satyavrata and the Saptarishis what is known as “Matsya Purana.”. Thus, Vishnu saved true devotees from destruction and endowed them with divine knowledge. During the 7 day waiting period the Lord in his gigantic fish form sought after the demon Hayagriva, killed him in a straight fight and retrieved the Vedas.