Fish is extremely perishable, so you want to be sure it’s fresh when you buy it. One clue to freshness is the overall condition of the fish counter. The air around it shouldn’t smell fishy. If it does, shop elsewhere. Fish fillets should be displayed on but not in direct contact with ice. Ask to inspect a fillet close up. It should smell fresh (not fishy), and it should look shiny, moist, and plump. Avoid fish that looks dull or spongy or with flesh that gapes apart.
Once you buy your fish, you want to keep it as cold as possible. Try to make the fish counter the last stop on your list. If it’s hot out or if you won’t be going straight home, ask the market to pack your fish with some ice. You should eat fish the same day you buy it, but if you don’t, put some ice in a colander set over a bowl to catch melted water and put the package of fish on the ice. Store the whole set-up in the back of your refrigerator on the bottom shelf for no more than a day or two.