Tuna sandwiches are a staple for most households -- and for good reason. They're an easy, tasty source of protein, and if you go light on the mayonnaise, they can be very low in fat. However, while canned tuna is low in mercury compared to other fish like swordfish and halibut, eating too much of it can still put you over the recommended levels, especially for younger children. A 40-pound child's tuna consumption shouldn't be more than a can every 11 days for chunk light and five weeks for white albacore [source: NRDC]. The solution? Blend sardines into your tuna salad. Sardines are inexpensive and high in omega-3 fatty acids, and they've got a much lower mercury content than larger fish.