Rummaging through a well-used freezer is rather like that proverbial box of chocolates because you never know what you'll get. Foods tend to get lost in the depths, so it's not uncommon to pull out a pack of frozen fish that's out of date. Frozen food won't spoil, as such, but it may or may not be worth eating.
It Starts With Freshness
How long your fish will last is determined by a few things, but one of the most important is how fresh it was from the start. If you're buying portions or fillets that were blast-frozen on a factory ship shortly after being caught, or on shore as soon as they were landed from a day-fishing boat, the process captured them at their absolute freshest.
On the other hand, fish three days out of date before it ever gets into the freezer is close to its limit already. By the time it's thawed, it may have off flavors or even be outright unsafe to eat. To complicate things further, unless you live near the coast, that piece of "fresh" fish was likely previously frozen, and then thawed at the store. You can be reasonably certain that a well-run market would thaw the fish properly, but since you didn't do it yourself, you can't be absolutely sure.
Frozen Fish Packaging Matters a Lot
The next factor that's important to your storage life is how the fish is packaged. Keeping air from contacting the fish is your biggest concern because it's air inside the packaging that causes "freezer burn." The moisture in your fish evaporates from its surface in the form of ice crystals, which basically freeze-dries a tough, tasteless patch at the surface of the fillet. Oxygen in the air also reacts with the proteins and especially fats, changing them chemically and creating unpleasant, off flavors.
Vacuum-packaged fish lasts the longest because the air is sucked right out of the package. If you don't have a vacuum sealer, you can use regular freezer bags but squeeze out as much air as possible – or suck it out through a straw – to minimize the presence of air. Some fish packers deliberately coat the individual portions with a protective layer of ice, which seems odd but protects it very effectively from exposure to outside air.
Your worst option is to freeze the fish directly in its supermarket-style foam tray covered with cling wrap. The packaging traps a large quantity of air inside the package and cuts the useful life of the fish sharply.
Freezer Storage Life of Fish
A freezer keeps your foods safe to eat, in the medical sense, indefinitely. What it loses isn't food safety – if it was safe when you froze it, it's still safe now – but taste and quality. For fish frozen on a supermarket tray, or packaged hastily in a loose bag, those might be gone forever in as little as 2 to 3 months.
Fish you've vacuum-sealed at home can outlast that easily with a useful life ranging from 6 months to a year or longer. Fish that's vacuum-sealed at the processing plant has had even less exposure to air and since commercial equipment makes a better vacuum than home sealers, those portions can often be kept up to twice as long. For vacuum-packed fish, the use by date is definitely only a suggestion.
Using Frozen Fish Out of Date
Food dating isn't a question of food safety; it's a measure of how long it's likely to still taste good. Fish in your freezer for two years past its date can still be in perfect condition if it was vacuum-sealed, while poorly packaged fish can be past its prime in a matter of weeks.
If you're uncertain, use your judgment. Fish that's visibly discolored, or has ice crystals all over it, or smells funky, should probably be discarded. If you've thawed the fish already, you have the option of slicing off a small piece and cooking it in the microwave. If it smells or tastes off, discard the rest.
If the fish looks and smells fine, you're good to go. Once thawed, you can use frozen fish in all the same ways you'd use fresh. Thawing overnight in the fridge is best, but you can thaw fish safely in cold water as long as you cook it immediately afterward. Don't use the microwave to thaw fish if you can help it because the microwave creates overcooked spots. It's better to cook directly from frozen if you're pressed for time, and just extend the cooking time by about half.