If you plan on eating a whoooooole lotta fish for protein, such as me (I eat it about 2 or 3 times a day one day, then the next day is all chicken), you should look into eating generally smaller fish, not larger ones such as salmon because they usually contain higher sources of mercury in them because they eat other smaller fish.
I eat Talipa. They're cheap, and good. You can also buy fish anywhere.... =P