Exodus 7:20-21

20, 21. And all the waters that were in the river were turned to blood Also, by implication, (Exodus 7:19,) all the waters that had been drawn from the river into the ponds, tanks, etc., underwent the change. The sweet, beneficent Nile water became red and putrid like stagnant blood, so that it poisoned the fishes and became unfit for use. The red moon of the eclipse is said to be turned into blood, Joel 2:3. Only the Nile water was smitten, for water could yet be obtained from the wells and by digging, as we see from Exodus 7:24.