Exodus 7:18

And the fish that is in the river shall die, and the river shall stink; and the Egyptians shall lothe to drink of the water of the river.

Cross-reference

Exodus 7:21 fulfills this prediction—fish die, Nile stinks, water undrinkable—confirming the plague's arrival.

Exodus 7:24 shows the aftermath—Egyptians digging for water—demonstrating the effect of the stinking Nile.

Exodus 7:20 records Moses striking the Nile to turn it to blood, which causes the fish death and stench here.