Charles Ellicott Commentary Exodus 7:18

"And the fish that are in the river shall die, and the river shall become foul; and the Egyptians shall loathe to drink water from the river." — Exodus 7:18 (ASV)

The fish that is in the river shall die. —The natural discoloration of the Nile, whether by red earth or by Cryptogams and Infusoriæ, has no harmful effect at all on the fish. Nor is the water made unfit for use at all by these discolorations. The Nile naturally abounds with various kinds of fish. Although most of them have an insipid taste to Europeans, yet in both ancient and modern times the subsistence of the natives has largely been drawn from this source. It was a severe punishment for the Egyptians to be deprived of their fish supply. It also implied contempt regarding their religious worship, since at least three species of Nile fish were sacred—the oxyrhineus, the lepidotus, and the phagrus, or eel. (Herodotus 2.72; Plutarch, De Iside et Osiride 7.18, 22.)

The river shall stink. —The Nile is said to sometimes have an offensive odour naturally. However, this phenomenon is not pronounced and can hardly be what is alluded to here. Rather, the text refers to the time when the blood-like waters, laden with the bodies of putrid fish, caused an unspeakable disgust and horror.