וְהַדָּגָ֨ה אֲשֶׁר־בַּיְאֹ֥ר מֵ֙תָה֙ וַיִּבְאַ֣שׁ הַיְאֹ֔ר וְלֹא־יָכְל֣וּ מִצְרַ֔יִם לִשְׁתּ֥וֹת מַ֖יִם מִן־הַיְאֹ֑ר וַיְהִ֥י הַדָּ֖ם בְּכָל־אֶ֥רֶץ מִצְרָֽיִם׃
And the fish that were in the river died; and the river became foul, and the Egyptians could not drink water from the river; and the blood was throughout all the land of Egypt.
Read halakhic rulings (Jewish law) on Exodus 7:21: and classical halakhic sources.