How important were fish to the Aztecs, were they symbolic?

Yes, fish play a role in the Mexica creation story, as told in the Leyenda de los soles. Quoting from Karl Taube:-
’The rain god Tlaloc rules over the third creation, the sun of rain. This world is destroyed by Quetzalcoatl in a rain of fire - probably volcanic ash... The fiery rain magically transforms the people of this race into turkeys. The fourth sun, the sun of water, is presided over by the wife of Tlaloc, Chalchiuhtlicue, She of the Jade Skirt, the goddess of streams and standing water. A great flood destroys this world and its people are transformed into fish. So massive is the flood that the mountains are washed away, causing the heavens to crash down upon the earth...
’[Later,] from the bones of the fish people mixed with the penitential blood of the gods, the present race of humans are born.’
In the Maya creation story Popol Vuh too, the Hero Twins are born again as fish-men and go on to defeat the lords of Xibalba.

They used bag-shaped nets made of grass fibre to catch the fish found in Lake Texcoco (none of which were particularly large).

Main source:-
• Aztec and Maya Myths by Karl Taube, British Museum Press, 1993
The image, taken from Taube’s book, shows Tlaloc letting blood upon a fish-man in a scene that may illustrate an early version of the creation of people. It comes from a bas-relief sculpture from El Tajin, Veracruz.