Freezing foods will cause some foods to lose some nutritional value, but in some cases is better than canning and is certainly better than storing long term in a refrigerator where it will degrade and in some cases spoil. In fact some foods retain more nutritive value if frozen than if left in the refrigerator for 1-2 weeks.
The decision to freeze depends on how long the food needs to be stored before consumption. I freeze poultry and fish if I don't plan to cook it in 24 hours, but I never freeze fruit or vegetables.
Freezing isn't done to save food from losing nutrition. It's done to save food from being thrown away.
1) before freezing, dry thawed products with paper towels. Large fish pieces can be wrapped in wax coated paper (if needed.) More moisture means more damage. Do not freeze products in excess liquid, so drain.
2) if the product is already frozen, leave it frozen while still in it's original container and place in the freezer. If the amount is too much for one meal however and the package is not re-sealable, I eventually transfer the product to zip loc freezer bags.
3) use zip loc freezer bags and squeeze out as much air as one reasonably can. That greatly limits the amount of surplus moisture in the bag that will cover the product in ice, plus air is a waste of space in the freezer.
4) don't pack more in one bag than you plan to use. Thawing and re-freezing is not a good plan.
Also, plan on remembering when you bought stuff. Older stuff (months) can be used even if you cut off freezer burned pieces, but the taste degrades.
