Frozen Fish Can Be Stored Indefinitely
According to the USDA, frozen foods will keep indefinitely if stored in an airtight package at a constant temperature at or below zero degrees Fahrenheit. Freezing fish does not decrease its nutritional value. Freezer burn may dry the fish out in spots, but it is still safe to eat.
Long-term Freezing Affects Quality
Storing fish for an extended period of time in the freezer affects its quality. Raw fish will remain at a higher quality after it is frozen than cooked meats, because cooking drives off moisture. You should only be concerned about frozen fish if the package has ripped or opened, exposing it to air. If it has gone rancid, it will have a strong "fishy" smell.
The FDA and USDA both indicate that fish frozen at or below zero degrees Fahrenheit will be safe to eat--even if it is frozen for a while--as long as it remains in an air-tight package. However, the fish should not be discolored or smell funny. If your fish has green or yellowish discoloration or a pungent odor, it's time to discard it.