The fish ... - The Egyptians subsisted to a great extent on the fish of the Nile, though salt-water fish were regarded as impure. A mortality among the fish was a plague that was much dreaded.
Related Commentaries of Exodus 7:21
Exodus 7:21
21 And the fish that was in the river died; and the river stank, and the Egyptians could not drink of the water of the river; and there was blood throughout all the land of Egypt.