Exodus 7:22

22. And the magicians ... did so with their enchantments, &c.--Little or no pure water could be procured, and therefore their imitation must have been on a small scale--the only drinkable water available being dug among the sands. It must have been on a sample or specimen of water dyed red with some coloring matter. But it was sufficient to serve as a pretext or command for the king to turn unmoved and go to his house.

Robert Jamieson, A. R. Fausset & David Brown, 1871 · public domain

Verse 22 And the magicians - did so - But if all the water in Egypt was turned into blood by Moses, where did the magicians get the water which they changed into blood? This question is answered in Exo 7:24. The Egyptians dug round about the river for water to drink, and it seems that the water obtained by this means was not bloody like that in the river: on this water therefore the magicians might operate. Again, though a general commission was given to Moses, not only to turn the waters of the river (Nile) into blood, but also those of their streams, rivers, ponds, and pools; yet it seems pretty clear from Exo 7:20 that he did not proceed thus far, at least in the first instance; for it is there stated that only the waters of the river were turned into blood. Afterwards the plague doubtless became general. At the commencement therefore of this plague, the magicians might obtain other water to imitate the miracle; and it would not be difficult for them, by juggling tricks or the assistance of a familiar spirit, (for we must not abandon the possibility of this use), to give it a bloody appearance, a fetid smell, and a bad taste. On either of these grounds there is no contradiction in the Mosaic account, though some have been very studious to find one. The plague of the bloody waters may be considered as a display of retributive justice against the Egyptians, for the murderous decree which enacted that all the male children of the Israelites should be drowned in that river, the waters of which, so necessary to their support and life, were now rendered not only insalubrious but deadly, by being turned into blood. As it is well known that the Nile was a chief object of Egyptian idolatry, (See Clarke's note on Exo 7:15), and that annually they sacrificed a girl, or as others say, both a boy and a girl, to this river, in gratitude for the benefits received from it, (Universal Hist., vol. i., p. 178, fol. edit)., God might have designed this plague as a punishment for such cruelty: and the contempt poured upon this object of their adoration, by turning its waters into blood, and rendering them fetid and corrupt, must have had a direct tendency to correct their idolatrous notions, and lead them to acknowledge the power and authority of the true God.

Adam Clarke, 1831 · public domain