Acne is an inflammatory condition. Research has shown that fish oils are known for decreasing inflammation and increasing hydration in the body. Fish oils are omega-3 fatty acids that help keep the skin smooth, healthy, clear and moisturized from the inside. We need to obtain omega-3 fatty acids from food, oils, or supplements because our bodies cannot make them on their own.
Omega-3 supplements has been shown to help those with acne because it reduces inflammation. Those with acne have high levels of inflammation and too much inflammation makes it easier for acne bacteria to turn into a clogged pore. A research from 2014 showed that supplementing with omega 3 or gamma-linolenic acid (GLA) reduces inflammatory acne by 40 to 50%.
The three main types of omega-3 fatty acids are α-Linolenic acid (ALA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA). ALA is found in plant sources (flaxseeds, chia seeds, hemp seeds) while EPA and DHA are found in algae and fish. ALA can convert into DHA EPA in the human body, but in limited amounts.
Cod liver oil has been shown to be the most effective source of omega-3 fatty acids and has the additional benefit of generous amounts of vitamin A and D, which are also good for the skin.
Not all fish oil supplements are created equal - there are cheap variations that may actually be worse for your skin than not taking the supplement at all. Look for a logo from the International Fish Oil Standards (IFOS) to ensure that you're not taking something fishy.
Y. Jung et al. Effect of Dietary Supplementation with Omega-3F Acid and gamma-linolenic Acid on Acne Vulgaris: A Randomised, Double-blind, Controlled Trial. 2014.