a vacant, lifeless stare—literally 'eyes of a dead fish,' used to describe someone who looks listless or expressionless.
徹夜明けで彼はまるで死んだ魚の目をしていた。
After staying up all night, he had the eyes of a dead fish.
急にあの子の目が死んだ魚の目になったのが心配だ。
I'm worried because that child's expression suddenly became vacant.
試験が終わったら皆、しばらく死んだ魚の目だった。
After the exam, everyone had lifeless looks for a while.