Click on this link to open the Healthy Seafood Guide, compiled by Tim Fitzgerald, a marine scientist for the Oceans program of the nonprofit organization Environmental Defense. In it, you'll find suggestions for how often to eat the most common fish, based on mercury levels, and whether the way they were farmed or caught damages the oceans. (Some fish, like salmon, tuna, and shrimp, appear several times on the list, depending on variety and origin.) Print it out, fold it, and put it your pocket; you'll have a quick reference when you're choosing your seafood. And if you aren't sure where your fish comes from, ask the grocer or restaurant waiter.
To get more in-depth info and tips on specific fish, go to environmentaldefense.org/seafood.