Those taking fish oil for the many apparent benefits of Omega 3 fatty acids should, according to this study, consider taking at least 200 I.U. of Vitamin E to counteract peroxidation leading to free radical production. I've also seen studies recommending small doses of Vitamin C as well, as C and E address water and fat soluble effects respectively. Turmeric appears to be more potent that either of the vitamins in this regard. And, there are a few brands of fish oil that test and control not only for pollutants, but for levels of peroxidation in their products, such as "Nordic Naturals," and "Eskimo 3." It's unclear whether or not the small amounts of C and E in these otherwise pure fish oil products is adequate to counteract the peroxidation effect in most people, and so the additional vitamins may be necessary.
Here is some description of stable vs. unstable fish oil. Whereas most brands control for impurities, not all, or perhaps only a few, control for oxidation of the oil:
Example of how herbs can be used to counteract the oxidizing effects of fish oil, where even stable fish oil may lead to slight increases in LDL while increasing good HDL and lowering VLDL:
The abstract below shows a moderate amount of garlic (probably 1 capsule 3x day ) effective when the intake of fish oil is very high (12 grams; the usual dosage, unless doctor recommended for a specific disease, is 2-4 grams of oil daily):
