Buying Fish

The Aish Rabbi Replies

One may not purchase cut fish from a store that sells non-kosher fish, even though the fins and scales of the fish that you want to buy are recognizable. The knife used may have residue from a non-kosher fish, and the proprietor cannot be relied upon that the knife is used exclusively for kosher fish.

Also, if the fins and scales are not recognizable, the fish pieces may be from a non-kosher fish.

One who has no alternative to purchasing cut fish from a non-kosher store should purchase only whole fish, have the fish cut in his presence, or purchase only pieces that have recognizable scales. If the fish was cut in the store, the cut pieces should be thoroughly washed and the cut surfaces lightly scraped with a knife edge.

(source: Teshuvos Chavas Yair 179; Laws of Kashrus – artscroll.com)