Hold the bag containing frozen fish under a running water tap. The water should be cold enough, to keep the fish raw. Never hold the fish directly under the running tap because you may end up making a slimy mess. Keep the bag sealed tight so that the water does not enter inside. Hold the fish for 15-20 minutes under the slow running tap. When you can feel the separate pieces, the fish is thawed. If it is one big piece, switch sides under running water and let it thaw from all sides.
Whenever you plan on cooking the fish, always take out the fish from the freezer and leave it in the fridge for at least 8 hours or overnight. Fish can go bad on the counter and make the kitchen smell awful if left there to thaw at room temperature. When thawed at cold temperatures, the fish retains its texture and shape.
If you want to cook fish in a few hours, leave the fish directly in a bowl of cold water. You can put the bag of fish inside the cold-water bowl or remove the bag. Leave the fish for an hour or two and then check if it’s thawed. Cook it as soon as it's thawed. If you don’t plan on cooking it for another few hours, place it in an airtight container and refrigerate.