Seeing as the object of the plague was to strike the Egyptians' god, what is the significance of the fact that the fish would die?
Seforno: To demonstrate that the water did not just resemble blood, but that it actually turned into blood. 1
In order to denigrate their god still further, by making the water putrid. 2
Something that the Egyptians were incapable of doing. Refer to 7:12:1:2, 7:22:2:1, and 7:23:1.1:3*.
And to render the water totally undrinkable - as the Pasuk goes on to state.
What is the definition of "Dagah"?
Rashi (to Yirmeyah 6:7): It means 'a lot of fish.'
Nedarim 51b: We can extrapolate from this Pasuk that "Dagah" incorporates both large and small species of fish. 1
Nedarim 51b: Nevertheless, in the realm of Nedarim, we follow Lashon Bnei Adam (commonly-spoken terminology, vernacular).
What is the meaning of, "v'Nil'u Mitzrayim Lishtos Mayim ..."?
Rashi: It means that the Egyptians will weary themselves trying to find a remedy to cure the waters of the Nile, to make them fit for drinking. 1
Seforno: They will attempt to find water by digging around the Nile.
Rashbam: They will realize that they are unable to drink, so they will stop trying - as the term means in Bereishis 19:11.
Targum Yonasan: "They made themselves weary;" as the term means in Bereishis 19:11.
Gur Aryeh: Rashi is explaining, usually a person becomes weary from exhausting toil; how would they become tired from the act of drinking itself? Rashi explains, they would exhaust themselves trying to fix the situation.
What is the significance of the various forms of punishment mentioned here?
Oznayim la'Torah: The Torah mentions four punishments, in fulfillment of the four parts that comprise each Makah (as stated by R. Eliezer in the Hagadah) - a. The water took on the appearance of blood; b. The fish died; 1 c. the river stank; d. There was a lack of water (and they had to dig to try and find some). 2
