I used to avoid fish altogether partially die to the sustainability issues and partially due to toxicity issues. As time went on I made an effort to become more educated on both of these issues as they pertain to all foods in general but especially sea food.
One way to keep aware of potential health and environmental issues as they pertain to seafood is to use references provided by the Monterey Bay Aquarium who helpfully present recent findings in a convenient pocket guide. Another is the The National Institute for Occupational Safety and Health (NIOSH) pocket guide and there are many more available resources including thankfully, at least two that I know of that are available as smart phone apps that are regularly updated.
Today I also ran into a well organized, visual overview that explains the risks and how to avoid them. I’ve presented that infographic below. Remember also to search this site for other ways to avoid mercury and heavy metal exposure and more importantly, how to safely test for and remove these toxins if you have been exposed.