Exodus 7:22 - Ellicott's Commentary On The Whole Bible

The magicians... did so with their enchantments. — The act of the magicians must have been a very poor imitation of the action of Moses and Aaron. The two brothers had turned into blood all the waters of the river, the canals, the pools or lakes, and the reservoirs. The magicians could not act on this large scale. They could only operate, or seem to operate, on some small quantity of water, obtained probably in the way noticed in Exodus 7:24. On this they succeeded, so far as to satisfy Pharaoh, who was probably easy to satisfy, and perhaps so far as to satisfy the courtiers. They turned the liquid of a red colour, or by sleight-of-hand substituted blood for it. The result was subjected to no test, and was perhaps not even done in the presence of any hostile witness. But it enabled the king to harden himself, and refuse the request of the brothers.