We had a whole debate here about fish oil vs flaxseed oil. I took the flaxseed oil side. There are pluses and minuses to each. Whichever you take, make sure it is refrigerated, and that the fish oil is tested to be mercury free(both oils in capsules can become rancid much too easily at room temperature). I think fish oil is also beneficial, but that I prefer to get my fish oil from eating oily fish low in mercury such as sardines, herring, or salmon. Others think that eating even very low mercury fish is a bad idea. I eat just small amounts(less than 14 ounces a week) of very low mercury fish. Check out this website to see how low the mercury level is in sardines(perhaps 1/20 th that of typical tuna?)

http://www.gotmercury.org/article.php?list=type&type=75

If you are that worried about mercury in fish oil, you could get salmon oil. Salmon tends to be very low in mercury and also has the highest amounts of omega 3 of all the fish oils. The downside? Salmon oil is more expensive than oil made from cheaper fish(I wonder if sardines and mackerel are often used to make the cheaper fish oils.)