Albert Barnes Commentary Exodus 7:21

"And the fish that were in the river died; and the river became foul, and the Egyptians could not drink water from the river; and the blood was throughout all the land of Egypt." — Exodus 7:21 (ASV)

The fish ... - The Egyptians largely depended on the fish of the Nile, though saltwater fish were considered impure. A widespread death of fish was a plague that was greatly feared.