If you aren't going to use fish within two days of buying it, by all means freeze it. To freeze fish, take it out of its wrapper, gently rinse it in cold water and pat it dry. For the very best results, wrap each fish fillet or steak separately in plastic wrap. Don't be stingy with the wrap: the closer to the fish and the more airtight it is, the better the fish will keep. After wrapping, place the fish in a freezer plastic bag, getting as much air out of the bag as possible. And don't forget to label the bag with the name of the fish, how much is in the bag, and the date you are putting it in the freezer.
Fatty fish like salmon will keep fairly well for 2 to 3 months, while lean fish stays longer‚Äîup to 6 months. But keep in mind the fish won't make you sick if kept longer, it just won't have as nice a flavor and texture. And the fresher it is when you freeze it, the better it will be when you thaw it.
Recipe: Salmon Mini Fillets