Seeing as the object of the plague was to strike the Egyptians' god, what is the significance of the fact that the fish died?
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
What does "Venil'u ... " mean?
Rashi: It means that the Egyptians will try to find a remedy to cure the water.
Seforno: They will attempt to find water by digging around the Nile.
Rashbam: Refer to Bereishis 19:11:1:1.
Targum Yonasan: Refer to Bereishis, 19:11:1:2.
What is "Mimenu" coming to preclude?
Torah Temimah (citing Zevachim, 29a): It comes to preclude a Machshavah of Chutz li'Mekomo from Kares. 1
The Korban is nevertheless Pasul.
Rashi writes that improper intent makes Pigul. Gitin 54b says "he heard him make Pigul.' This implies that it is through speech!
Moshav Zekenim: 'Machshavah' refers to speech, like 'ha'Choshev to be Shole'ach Yad' (Bava Metzi'a 43b).
Zevachim 29a learns from Mimenu that there is Kares only for a Korban offered with intent of Chutz li'Zmano, but not Chutz li'Mekomo. Perhaps it is the opposite; Kares is only for Chutz li'Mkomo!
Moshav Zekenim, from Zevachim 29a: We learn from a Gezeirah Shavah "Avon-Avon" from 19:8, which discusses Nosar, which resembles Chutz li'Zmano.