All fishes are highly perishable, especially once they have been cut. It is best to buy fish whole and have the retailer clean or fillet it while you wait, or prepare it yourself just before cooking. Generally, fish is best eaten within 24 hours of purchase. Oily fish go off more quickly than white fish because their oils oxidize, causing the flesh to go rancid. Fish with their guts intact also deteriorate quickly, so gut fish before storing.
To store fish, remove it from its wrappings, rinse it under cold running water and pat it dry. Then rewrap the fish loosely in a sheet of aluminum foil or a clean plastic food storage bag and store it in the coldest part of the refrigerator.