Check the date: Use or freeze by two days after the sell-by date. Cooked fish can stay in the fridge for 5-6 days past the sell-by date. Frozen fish can stay in the freezer for 6-9 months past the sell-by date. Touch the outside: Raw fish has gone bad may develop a slimy coating. Give it the smell test: If there is a very strong fishy smell, your fish has started to go bad. Toss it. Check for a milky color: For raw fish, it will begin to develop a shiny, milky color when it’s going off. Toss it if you notice this change.