Exodus 7:18
And the fish that is in the river shall die, and the river shall stink; and the Egyptians shall lothe to drink of the water of the river. And the fish that is in the river shall die, and the river shall stink; and the Egyptians shall lothe to drink of the water of the river.
The verse foretells what striking the river would do: the fish in it would die, the river would stink and become foul, and the Egyptians would loathe its water and refuse to drink it. This was announced to Pharaoh beforehand as a warning demanding that he let Israel go to serve God, so that by obeying he could avoid the judgments. The warning was to be spoken at the river's brink, calling for Israel to serve God in the wilderness, so that Pharaoh would know that the LORD is God.
The Egyptians worshiped the Nile as a god because they depended on it to quench thirst, to water their lands to make crops grow, and to supply fish and food and wealth for their tables, giving to the river the devotion belonging to God alone. Pharaoh went to the river daily, every morning standing on its banks to worship with praise and thanksgiving to the water. There he declared the river's great good and great power, saying that without it they could not exist because it watered their lands and supplied meat for their tables.
When Pharaoh refused to listen, Moses with Aaron struck the waters before Pharaoh and his attendants and great men and servants, and all the adored waters of the river were turned to blood. The waters lay as a putrid mass, foul and offensive from stench and unusable for any purpose. The Egyptians had to dig wells to get water. The plague on the waters lasted seven days.
The plague came upon one of Pharaoh's exalted gods and showed the Nile to be powerless. The revered fish died and putrefied and the water drunk with reverence was turned to blood, exposing the powerlessness of the Nile and its fish as objects of worship and showing that Jehovah could overthrow the gods in which Egypt trusted. Despite the warning and the seven days, Pharaoh did not heed or humble himself or soften, but hardened his heart. Pharaoh despised the words spoken to him, yet he had no power to harm the messengers.
The sign did not soften Pharaoh but increased his hatred of the Israelites. He believed the miracle had been performed by magic because the work of the magicians led him to that conclusion. The changing of the Nile to blood was a miraculous interposition of God for His chosen people and a calamity on the Egyptians for persecuting Israel, showing His protecting power and His hand holding back the evils around Israel. Israel should have remembered that protecting power and trusted when walled in at the Red Sea, instead of reproaching Moses and fearing death in the wilderness or by their enemies.