Purchasing Kosher Fish
Fish are some of the most accessible foods for the kosher consumer, since, unlike beef and poultry, they do not require any halachic preparations prior to their processing [cooking]. It is also rather simple to determine whether or not a fish is from a kosher species. The Torah, in Vayikra 1:9, informs us that a kosher fish is one which has both fins and removable scales (point of interest: every fish which has easily removable scales will also have fins).
Whole fish and fish steaks should preferably be purchased from a store which has reliable Rabbinical supervision. The reason for this is because non-kosher stores which sell fish commonly use one knife to cut different species, kosher and non-kosher alike. Therefore, residue from the slicing of non-kosher fish may remain on the blade of the knife and subsequently be rubbed onto the cutting site of the kosher fish.
However, if no fish store is available under reliable Rabbinical supervision, then one may buy whole fish or fish steaks from any store, as long as one did not actually see the kosher fish soaking amidst non-kosher fish and some scales are still on the fish.
Prior to cooking the fish, one should take a straight-edged knife and, using only minimal pressure, scrape off the area where the store knife would have cut. This will wipe off any residue from the blade which cut the non-kosher fish. After this, thoroughly wash the fish.
Note: Fish which is already ground, cooked, smoked or canned must have reliable kosher supervision.
The following is a partial list of kosher and non- kosher species of fish: