What is the significance of "u'Ntashticha ha'Midbarah"?
Rashi: This is extermination of fish. When they are spread on the land, they die.
Radak: Ha'Midbarah is a Mashal. Fish cannot live there. So the Egyptians will die via the sword on the field when they leave to fight the Kasdim. They will not be buried; Chayos and birds will eat them.
Malbim: There his army will scatter, and fall on the field. They will not fight in the city and fortress.
Why does it say "Lo Se'asef v'Lo Sikavetz"?
Targum Yonasan: They will not be gathered and not buried. Radak ? i.e. they will not be gathered for burial.
Malbim: This refers to the Mashal of fish. First they gathered them, and afterwards bring them into the house. They will not be brought into the house; further, they will not even be gathered, for Chayos and birds will eat them.