Turned to blood - This miracle would bear a certain resemblance to natural phenomena, and therefore be one which Pharaoh might see with amazement and dismay, yet without complete conviction. It is well known that before the rise the water of the Nile is green and unfit to drink. About the 25th of June it becomes clear, and then yellow, and gradually reddish like ochre; an effect due to the presence of microscopic cryptogams and infusoria. The supernatural character of the visitation was tested by the suddenness of the change, by its immediate connection with the words and act of Moses, and by its effects. It killed the fish, and made the water unfit for use, neither of which results follows the annual discoloration.
Related Commentaries of Exodus 7:17
Exodus 7:17
17 Thus saith the LORD, In this thou shalt know that I am the LORD: behold, I will smite with the rod that is in mine hand upon the waters which are in the river, and they shall be turned to blood.