Midrash su Esodo 7:19

And the Lord spoke unto Moses (Exod. 7:19). Scripture says elsewhere in allusion to this verse: Surely oppression turneth a wise man into a fool; and a gift destroyeth the understanding (Eccles. 7:7). Surely oppression turneth a wise man into a fool implies that when a wise man concerns himself with many matters, they confound his wisdom. And a gift destroyeth the understanding indicates that understanding of the Torah, which was placed as a gift in the heart of many (is destroyed). Oppression (osek) turneth a wise man into a fool implies that the scholar who occupies (ashak) himself (overmuch) with community problems forgets his learning. R. Johanan the son of Levi said: R. Judah the son of Pedayah taught me sixty laws concerning a grave which has been plowed over, and I have forgotten all of them because I occupied myself with the needs of the community. Hence, oppression turneth a wise man into a fool. Oppression turneth a wise man into a fool. The attention that Solomon devoted to insignificant matters led him astray. For it came to pass, when Solomon was old, that his wives turned away his heart with other gods (I Kings 11:4).

...Rabbi Aveen, the Levite, son of Rabbi, said: "From the plague of blood, the Israelites got rich. How? An Israelite and and Egyptian living in one house and a barrel was full of water; the Egyptian would go to fill the jug from it and it would be filled with blood. The Israelite would drink water from the barrel, and the Egyptian would say, 'Give me a little water in your hand' and he would give him, but it turned into blood. And he would say, 'You and I both drink from the same platter!' The Israelite would drink water and the Egyptian blood, but when he would purchase it from an Israelite with money, he would be drinking water. From here, the Israelites became rich."

Get thee unto Pharaoh in the morning; lo, he goeth out unto the water (Exod. 7:14). Thus the Lord said: Through this (water) you shall cause him to know that I am the Lord. I will smite the water of the Nile with the staff that is in your hand, and it shall turn to blood. Why were the waters turned to blood first? Because Pharaoh and the Egyptians worshipped the Nile, and the Holy One, blessed be He, said: Smite his god first and then him, as in the popular aphorism, “Strike the gods and the priests will be frightened.” And the Lord said unto Moses: “Say unto Aaron: Stretch out thy hand, and smite” (Exod. 7:19). Smite the water of the Nile, the roots of the plants in the rivers, and indeed, every source of their water, even that which is in their ladles. And blood appeared in the wood and stone, and even the spittle of the Egyptians turned to blood.