There are all kinds of warnings (health and environmental) about certain fish, i.e. farm-raised Salmon, Asian Tilapia, Bluefin Tuna and the like. So it’s a common enough question put to rabbis: ‘Is it permissible to buy a fillet of kosher fish from a local fish store or supermarket?—as the latter typically have a far a wider selection than those of a kosher market. (For those who live in areas without a proliferation of kosher markets—this may be a basic question of survival.)
The overriding concern is that the fish vendor who cut your salmon may not have cleaned the knife or counter beforehand and some shmaltz from say, Shark or Shellfish, may have found its way onto your fresh filet of Sole. (Yuck!) Obviously, some vendors are very clean, thus the chances of this happening are quite slim. As for those who are less than hygienic, a call to the local health inspector may be more effective and appropriate than a phone call to the rabbi.
In any event, after purchase one must simply rinse the fish well under the kitchen faucet. (Hebrew: “shifshuf v’hadacha”) Thus if any foreign particles did find its way onto your fish, it will be flushed down the drain.
[For those who wish to review the matter in the Shulchan Aruch and relevant commentaries, here are the appropriate sources: Shach YD 96.16; Torat Chatath 61:9; Aruch HaShulchan YD 96.21; Pitchei Teshuva YD 96.5; Shut Chovoth Yair #179. Furthermore, as raw fish, like raw meat, is rinsed for hygienic purposes before cooking, the case is very much comparable to that found in the Shulchan Aruch YD 91.2. There, consent is granted to store cold raw meat in a non-kosher pot, as one will inevitably wash the meat before cooking.]
One should mention the fastidious custom of those who lightly scrape the cut edges of the fish with a knife. (Cf. Chovoth Yair #179) Additionally, there are those who will even bring their own knives and cutting boards to the fish store. May they be rewarded for their piety…
Just a reminder. If you purchase fish that is not readily recognizable by its distinguished color and tone (that is species other than Salmon or Tuna), be sure to buy fish with some skin still attached, so that you can verify that the fish has scales and is therefore from a kosher species. For a list of Kosher and non-Kosher fish, click here.