Albert Barnes Commentary Exodus 7:17

"Thus saith Jehovah, In this thou shalt know that I am Jehovah: behold, I will smite with the rod that is in my hand upon the waters which are in the river, and they shall be turned to blood." — Exodus 7:17 (ASV)

Turned to blood - This miracle would bear a certain resemblance to natural phenomena, and therefore be one that Pharaoh might see with amazement and dismay, yet without complete conviction. It is well known that before the river rises, the water of the Nile is green and unfit to drink. Around June 25th, it becomes clear, 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 follow the annual discoloration.