How can it be explained to the customer that the weight of frozen fish will decrease after thawing upon repackaging, and is it sufficient to write "Net weight approximately 1 kg" or "Gross weight includes frozen preservative"?

The aforementioned sale is valid because the weight is not unknown, whether the seller writes the net weight or the gross weight on the box. The weight is known and the price is known, and this is sufficient for the validity of the sale.