John Calvin Commentary Psalms 105:29

"He turned their waters into blood, And slew their fish." — Psalms 105:29 (ASV)

He turned their waters into blood. The severity of this plague to the Egyptians can be inferred from the fact that the element of water is one of the two great means of supporting life. And the power of God shines forth more brightly from the fact that, although the land of Egypt is well irrigated, the Egyptians were parched with thirst amidst an abundance of water.

It is later said that frogs were brought forth, and entered even into the chambers of the kings; by which God clearly showed that He was the author of the miracle. For although all Egypt swarmed with frogs, the courts of the kings should have been exempt from this nuisance.

By the term kings, is meant either the nobles of the realm or the king’s sons, who were brought up in the expectation of royal power; for at that time, as is well known, only one king reigned over all Egypt.

From this we learn how easily, and almost mockingly, God humbles those who pride themselves in the flesh. He did not gather an army to fight against the Egyptians, nor did He immediately arm His angels or thunder from heaven. Instead, He brought forth frogs, which contemptuously trampled upon the pride of that haughty nation, who held the rest of the world in contempt. It would have been no disgrace for them to have been conquered by powerful enemies; but how dishonorable was it to be vanquished by frogs? God thus intended to show that He has no need of powerful armies to destroy the wicked; for He can do this, almost playfully, whenever He pleases.