I have copied the information from the link above in case anybody had any difficulty in opening it.
Tilapia is in the first group.
Freshwater farmed fish may not have the same level of important minerals like selenium or fat soluble vitamins as marine fish.
There are questions as to farmed fish containing other pollutants too.
They are improving the Omega 3:6 status of farmed salmon but we need better labelling to know how much Omega 3 and 6 farmed fish contain.
Farmed fish may also contain less fat soluble vitamins including vitamin D.
http://www.nrdc.org/health/effects/mercury/guide.asp
ASTRACT
"LEAST MERCURY
Enjoy these fish:
Anchovies
Butterfish
Catfish
Clam
Crab (Domestic)
Crawfish/Crayfish
Croaker (Atlantic)
Flounder*
Haddock (Atlantic)*
Hake
Herring
Mackerel (N. Atlantic, Chub)
Mullet
Oyster
Perch (Ocean)
Plaice
Pollock
Salmon (Canned)**
Salmon (Fresh)**
Sardine
Scallop*
Shad (American)
Shrimp*
Sole (Pacific)
Squid (Calamari)
Tilapia
Trout (Freshwater)
Whitefish
Whiting
MODERATE MERCURY
Eat six servings or less per month:
Bass (Striped, Black)
Carp
Cod (Alaskan)*
Croaker (White Pacific)
Halibut (Atlantic)*
Halibut (Pacific)
Jacksmelt
(Silverside)
Lobster
Mahi Mahi
Monkfish*
Perch (Freshwater)
Sablefish
Skate*
Snapper*
Tuna (Canned
chunk light)
Tuna (Skipjack)*
Weakfish (Sea Trout)
HIGH MERCURY
Eat three servings or less per month:
Bluefish
Grouper*
Mackerel (Spanish, Gulf)
Sea Bass (Chilean)*
Tuna (Canned Albacore)
Tuna (Yellowfin)*
HIGHEST MERCURY
Avoid eating:
Mackerel (King)
Marlin*
Orange Roughy*
Shark*
Swordfish*
Tilefish*
Tuna
(Bigeye, Ahi)*
* Fish in Trouble! These fish are perilously low in numbers or are caught using environmentally destructive methods. To learn more, see the Monterey Bay Aquarium and the Blue Ocean Institute, both of which provide guides to fish to enjoy or avoid on the basis of environmental factors.
** Farmed Salmon may contain PCB's, chemicals with serious long-term health effects.
Sources for NRDC's guide: The data for this guide to mercury in fish comes from two federal agencies: the Food and Drug Administration, which tests fish for mercury, and the Environmental Protection Agency, which determines mercury levels that it considers safe for women of childbearing age.
About the mercury-level categories: The categories on the list (least mercury to highest mercury) are determined according to the following mercury levels in the flesh of tested fish.
* Least mercury: Less than 0.09 parts per million
* Moderate mercury: From 0.09 to 0.29 parts per million
* High mercury: From 0.3 to 0.49 parts per million
* Highest mercury: More than .5 parts per million"