10 Fish You Should Avoid (And Why)

Imported Catfish may contain antibiotics banned in food in the U.S.
Farmed Eel is potentially high in PCBs and mercury.
King Mackerel is high in mercury.
Orange Roughy is high in mercury and overfished.
Chilean Sea Bass is overfished and may contain mercury.
Shark is high in mercury.
Imported Shrimp may contain antibiotics and chemical residue.
Swordfish is high in mercury.
Tilefish is high in mercury.
Bluefin Tuna is overfished and may have more mercury than other
types of tuna.
Sneaky Swaps
Recent reports found these substitutions to be among the most
common. While they're generally harmless from a health standpoint,
you're paying an inflated price- and getting a fish that has a stronger
flavor or a less pleasant texture than what you asked for.
If you order Sole, you might get Flounder; Red Snapper, you might get 
Tilapia; Halibut, you might get Striped Bass; Perch, you might get
Zander. So make sure you ask lots of questions before you buy.
-Health Magazine, July/August 2013