Fish has things in it like selenium, which attracts heavy metals like mercury. Thus, if you take fish oil that already has mercury in it, then that is a problem. But, if you get fish oil that is from non-contaminated waters or has been processed to remove the mercury, then you get the benefits of fish oil, helping you detox, rather than it adding toxins. Thus, the trick is taking fish oil that is not contaminated. There are several brands that either process the mercury out or test the fish oil for mercury on a regular basis to make sure the fish they use is not contaminated. This fish would have to be caught in only a few select water bodies, since most water is now reported to be contaminated. We have taken fish oil, and now take cod liver oil. This difference is very noticeable when we take these oils. My children are recovered from autism, but it took much more than cod liver oil/fish oils. But, the oils did definitely help in decreasing ADHD symptoms. I love it and still take it. The best brand on the market is currently said to be Carleson's. You can go to www.heidinotes.com to learn more of my views on autism, ADHD, etc.