All commercial caught food have to be dipped or treated with a chemical to destroy parasites. It is sprayed on while they are being rinsed for packing. I will try to find the name of it for you and post it up here. The way to get around that chemical is to soak your seafood in fresh water (very cold) and it should be fine. The chemical loses its grip on the food when it thaws and the cold soak causes it to drop to the bottom of the container. Sulfur Dioxide is often used as a fumigant to contral the smell in seafood as well. Two examples are the commonly used preservatives butylated hydroxyanisole (BHA) and sulfites. Sulfites can be used on seafood to help restore its fresh look, adding these to red meats is illegal in the US. BHA and the related compound butylated hydroxytoluene (BHT) have been used for years, mostly in foods that are high in fats and oils. They slow the development of off-flavors, odors, and color changes caused by oxidation. Studies have shown that at very high levels in the diets of laboratory animals, BHA could cause tumors in the forestomach of rats, mice and hamsters, and liver tumors in FISH. You might want to avoid foods that have these sulfites in them, sulfur dioxide, sodium sulfite, sodium and potassium bisulfite, sodium and potassium metabisulfite. Hope this helps.
I will try to find the link to the information and post it for you.