When Moses turned the Nile to blood, the fish died and a stench arose (Exodus 7:18, 21).  Nothing is said about fish or putrid smells in the account of Exodus in the Pentateuch.  When Isaiah recounts the exodus, however, he talks about the dying stinking fish: “Behold, I dry up the sea with My rebuke, I make the rivers a wilderness; their fish stink for lack of water
and die of thirst” (Isaiah 50:2).

Isaiah has conflated the first plague with the exodus at the sea.  Turning the Nile to blood is a preview of opening a way through the sea.  Both events leave behind dead fish, and both events leave people dying of thirst.  As Toby Sumpter has pointed out, this link is already implicit in the book of Exodus: The Nile is a river of blood even before it turns red, since it is full of the blood of Israelite boys.  Eye for eye, blood for blood: Yahweh turns the Red Sea to a sea of blood by drowning Pharaoh’s chariots.

Presumably, the fish at the Red Sea are the men of Pharaoh, himself a sea monster, the serpent of the Nile.