The only living fish demon was neither in Nanjiang County nor even in the State of Chu.
It was in a river basin on the border between the States of Chu and Wu.
"In the southeast basin of the Dajiang River, a red-scaled fish has attained sentience. This demon is named Jiang Chiyou. He once roamed the middle reaches of the Dajiang River, later joined Fox Origin, and settled in the Water Clam Palace. He has not appeared in the mortal world for nearly three hundred years..."
Chapter 490