The avatara of Paramathma as fish viz., Matsyavatara, can be found in 8th Skanda of Srimadbhagavatha. This happened during Chakshusha Manvantara.
First Story of Matsyavatara – Once Brahma, during his Dina pralaya, felt sleepy, and did not had the complete control over the Vedas and their Abhimani devatas. Hayagreeva named Daithya, utilising the opportunity stole the Vedas (with Abhimani devatas) and Paramathma incarnated as a fish (Matsyavatara) and killed Hayagreeva Daithya and saved the Vedas and brought back to Brahma.
Second part of Matsyavatara – Once Satyavratha Maharaja, (Satyavratha Maharaja later become Vaivaswatha Manu, Son of Surya) was doing penance without taking food & taking only water. He was doing Tarpana at Kritamala River, a small Fish come into his hand & he left that water along with the fish in the river. The fish requested that it may be kept in a safe place since there is danger of being swallowed by big fishes in the river. So the king kept it in his Kamandalu. The fish grew large by the hour so that the fish was transferred to a well the next day. Then the fish got large enough so that it was transferred to the pond, then in a river and at last in the sea. The king was surprised and prayed almighty to reveal the fun or magic behind it. They Lord Vishnu appeared and told Manu that he is Lord Vishnu. He further told that from today on the seventh day, the three lokas will be immersed in sea and that he will send one boat. Told the king to collect vegetables and other food materials and store in the boat alongwith Rishigalu. You tie the boat on the fish’s horns to protect from wind. I will be dragging that ship up to the end of Brahmas night. Then I will reply to your questions Then Lord Vishnu disappeared.
The upadesha given by Lord Vishnu in the roopa of Matsya (Fish) to Satyavratha is called as “Matsya Purana”