Matsyasana (fish pose) is one of my favorite asanas because it's so energizing and calming at the same time. It induces a feeling of joy and happiness, a readiness to take on whatever the day has in store. In yoga lore, matsyasana is said to be the destroyer of all diseases, and among its many benefits, counteracts fatigue and anxiety.
Even more interesting is its esoteric significance. In Light on Yoga, BKS Iyengar writes that matsyasana is dedicated to Matsyavatara,
"the fish incarnation of Vishnu, the source and maintainer of the universe and of all things. It is related that once upon a time the whole earth had become corrupt and was about to be overwhelmed by a universal flood. Vishnu took the form of a fish and warned Manu (the Hindu Adam) of the impending disaster. The fish then carried Manu, his family and the seven great sages in a ship, fastened to a horn on his head. It also saved the Vedas from the flood."
Other versions of the story invite comparisons of this flood with the biblical tale of Noah. Vishnu takes on the form of Matsyavatara during a deluge that submerges the Earth. A rishi gathers together in a boat two of each animal species, and the golden fish avatar of Vishnu then pulls the boat through the deluge to safety.
When perfoming matsyasana, try feeling the expansive opening of your heart as if you, too, are an incarnation of Vishnu, the preserver and restorer of the universe. God lives within you as you.