Shellfish is one of the most allergic types of food that most people don’t even realize they are sensitive to. Shrimp, crab and other sources of shellfish can cause acne if you’re sensitive to shellfish allergens. Perhaps you don’t break out in hives, but if you notice a rash or acne eruption on your skin and have eaten shellfish recently, then you may want to consider not eating shellfish to see if it improves. Shellfish is another source of iodine, which as you know now can cause acne flare-ups. Omega 3 rich fish, such as salmon, trout and halibut, seem to actually improve skin issues like acne, so consider eating more of these sources of fish instead of shellfish.