No rule for this one
No ending or word group predicts the article of Fisch. Learn it as a pair and say it out loud: der Fisch.
der Fisch
It's der Fisch. Fisch is masculine, plural die Fische.
Der Fisch ist frisch.
The fish is fresh.
No ending or word group predicts the article of Fisch. Learn it as a pair and say it out loud: der Fisch.