It’s a good source of protein. Some, like salmon and tuna, also give you omega-3s, which your body needs. But what about mercury and other contaminants? You can have cooked seafood twice per week. Each serving can be up to 6 ounces, which is the size of two decks of cards. Choose types that are lower in mercury, such as salmon, tilapia, and trout. Avoid shark, swordfish, king mackerel, and tilefish, which have high levels of mercury.