Which age is known as the "Age of Fish"?

The age known as the "Age of Fish" is the Devonian Period.

The Devonian was a geologic period that lasted roughly from 419 to 359 million years ago. It was the fourth period of the Paleozoic Era and followed the Silurian Period.

Fish, particularly jawed fish, became highly diverse during the Devonian. Armored placoderms dominated many aquatic environments, while early cartilaginous and bony fishes also expanded significantly.

The Devonian also saw the appearance and diversification of fish lineages related to modern groups. Lobe-finned fishes began giving rise to early tetrapods, the ancestors of four-limbed vertebrates. Because of this exceptional fish diversity and evolutionary development, the Devonian is popularly called the Age of Fishes.