Matthew Poole and continuators on Exodus 7:19

19 And the LORD spake unto Moses, Say unto Aaron, Take thy rod, and stretch out thine hand upon the waters of Egypt, upon their streams, upon their rivers, and upon their ponds, and upon all their pooles of water , that they may become blood, and that there may be blood throughout all the land of Egypt, both in vessels of wood, and in vessels of stone.

Not that he was to go to every pool to use this ceremony there, but he stretched his hand and rod over some of them in the name of all the rest, which he might signifie either by his words, or by the various motions of his rod several ways.

Marginal references: Heb. gathering of their waters.