A knife that was used for cutting a non-kosher fish should not be used for cutting kosher fish. Even though the flavor added by the knife, if it is detrimental to the taste, would not make the fish forbidden, in this case, it enhances the taste.
A servant of Rabbi Levi used to resell food products and was considered trustworthy. After Rabbi Levi died, Rabbi Yochanan said, "A servant of a trustworthy person remains trustworthy even after the master's death - unless proven otherwise." In the same way, the wife of a trustworthy person is just as trustworthy as her husband.
Art: Fishmonger's Stall by Adriaen van Utrecht