Fish that has been defrosted in the refrigerator will last for 1 to 2 days if kept in the refrigerator. If the fish is not cooked within 1 to 2 days, it can be safely refrozen.
The safest way to defrost fish is in the refrigerator. Depending on the size of the filet, it may take a day or more to thaw completely. Once thawed in this manner, it is safe to leave the fish for a day or two in the refrigerator before cooking.
Another safe way to thaw fish is via cold water thawing. While it is faster than defrosting in the refrigerator, it does require a bit more attention on the chef's part. First, be sure the filet is in an airtight plastic bag. Submerge the bag in a bowl of cold tap water. Never use hot water as that may cause harmful bacteria to appear. The water should be changed about every 30 minutes so that it remains at a cold temperature. Thawing time with this method is about 30 minutes per pound, and a small fish filet can be defrosted in about an hour or so. Food using the cold water method should be cooked immediately to ensure it's safe to consume.