Fish that has been defrosted in the fridge will stay good for one to two days if it is kept in the fridge. If you don’t cook the fish within one to two days, you can safely put it back in the freezer.
The fridge is the safest place to let fish thaw. Depending on how big the fillet is, it could take a day or more to fully thaw. Once the fish is thawed this way, it is safe to put it in the fridge for a day or two before cooking.
Fish can also be thawed in cold water, which is a safe way to do it. Even though it’s faster than defrosting in the fridge, the chef has to pay a bit more attention. First, make sure the fillet is in a plastic bag that can’t leak air.
Put the bag in a bowl of cold water from the tap. Never use hot water, because it could cause bacteria that are bad for you to grow. About once every 30 minutes, the water should be changed so that it stays cold.
With this method, it takes about 30 minutes per pound to thaw, so a small fish fillet will take about an hour. Food that was cooked with cold water should be cooked right away to make sure it is safe to eat.