The only criterion for fish to be kosher is that it have both fins and scales. Fish does not have to be slaughtered or salted as do meat and fowl. Kosher fish include cod, flounder, haddock, halibut, herring, mackerel, pickerel, pike, salmon, trout, and whitefish. Non-kosher fish include swordfish, shark, eel, octopus, and skate, as well as all shellfish, clams, crabs, lobster, oyster and shrimp. For a listing of kosher fish, see the Kosher Fish List.
The definition of fins and scales must be as designated by Jewish law. Not everything commonly called a "scale" meets the Torah's criteria. Therefore, it is best to buy fish from a merchant familiar with kosher fish types.
When buying fish, either buy it whole so that you can see the fins and scales, or, if the fish is sliced, filleted, or ground, buy only from a fish store that sells kosher fish exclusively. This will ensure that knives or other utensils are used only on kosher fish, and that no other mix-up can occur.
Packaged and canned fish, such as tuna and sardines, need reliable kosher certification. Smoked fish must also have certification to ensure that it was smoked only with other kosher fish, and that all other kosher criteria have been met.
What about frozen packaged fish that goes directly from sea to package and frozen. Like ahi tuna for example?
(No certification, no skin)
The only way to know for sure what type of fish it is, if it's without skin, is if it's certified.
I read that the way they catch the tuna in nets other small fish can get caught up too which is why it needs a hechsher.