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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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Sounds like a way of checking if the fish has scales or not. If it has scales (and of course fins) is is kosher. However, the fact that it has been cut with a probably non-kosher knife is a problem. – Epicentre Dec 24 '13 at 6:44

Rabbi Aryeh Citron notes on Chabad.org:

When purchasing fish from a store that also sells non-kosher fish, one should ask the fish monger to thoroughly wash the knife and board that he will use to fillet the fish. Some people prefer to bring their own knife and board for him to use.

If the fish monger filleted a kosher fish with a knife that was not properly washed, one must wash the area that was cut and scrape it off with a knife in order to remove any non-kosher residue.

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.

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