Fish will weigh less after it is thawed because it loses some water when it is thawed.
It depends what you cook, fish in the freezer, fully raw meats in the fridge etc.
Due to health risks, no meat (or fish) should be refrozen after it has been thawed.
Fish Stock
It can be, but it is not desirable to do so. The fish could not have been thawed for long and would have to have been thawed under refrigeration. Freezing it again will adversely affect the quality of the fish. You might consider cooking it first, then freezing it.
There are food safety issues with thawing at room temperature and it is not recommended. If it is in a sealed package, it could be quick-thawed under cool running water - then immediately cooked. Or it could be thawed in the microwave - and immediately cooked. Or it could be cooked from frozen. Should you choose to thaw by any means other than in the fridge, it should be cooked immediately upon thawing and not stored for cooking later.
because your cooking it so it doesn't matter as long as its not burnt
If you mean to fry it the oil should be 360 F before you drop in the fish. If you mean to bake it a 350F oven should work. The fish should reach in internal temperature of 145 or higher to be cooked through.
If they came in a box, I would read the directions on the box as for heating. The fish may already be cooked, especially if they are already breaded. If no directions came with the packaging, and the fish have been in the freezer for some time, I would find something else to eat. In my opinion, fish have to be flash frozen to keep the cells in the meat from breaking. When the cells break from slow freezing, the fish meat is 'mushy' and can 'go bad' quicker.
No, frozen is frozen. But you can refreeze something that has been thawed. You can refreeze thawed meat long as there are still ice crystals on it. Once completely thawed then you need to cook it first then refreeze. This applies to solid pieces only, ground meat once more than half thawed needs to be cooked first then refrozen always.
If you grill the fish too long, it will dry out drastically and won't be as good. You should time it so that the inside of the fish is flaky and not watery, and the outside is lightly browned. The timing and the temperature go hand in hand, so you should determine the correct temperature before putting your fish on the grill.
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