Title: Mercury and Fish Oil Omega-3 Supplements, Medscape
The last time I checked the EPA site it seemed all fish had some mercury. Some more than others.
I was concerned that useful omega-3 fatty acid supplements had mercury from the fish used for the supplement. Some calcium supplements have lead. I was concerned about a similar contamination with mercury.
In this article I had advanced testing done on the inside of the capsules and found that some brands had no mercury.
Full-text article: Schaller JL. Mercury and Fish Oil Supplements. Medscape General Medicine. April 13, 2001. Available at: www.medscape.com/viewarticle/408125.