Why this seafood is bad for you
The Monterey Bay Aquarium Seafood Watch looks at which fish are "fished or farmed in ways that have less impact on the environment." Basically, it's not cool to eat endangered species on land, so maybe don't eat the panda bear of the sea, either.
Then, the good folks at the EWG take the Monterey Bay data and overlay on top of it whether or not it's healthy to consume the selected fish, based on their mercury content. Mercury, according to the EWG/science, negatively effects your heart, nervous system, and general well-being.