I heard that a kosher type fish bought from a grocer that has also non-kosher fish, that scraping it with a finger nail where the blade cut makes it good to eat. What is the source for this idea?
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For the second paragraph, he cites Pische S'shuva 96:5 and adds in a footnote that "If one is unsure whether the fish was cut with a clean knife, one may be lenient and not wash the piece", referring the reader to SA and Rama 96:4 and Shach and Taz ad loc.
I haven't checked those sources.