D.K.
The website below lists the mercury levels in many fish and seafood. I think some of the worst are swordfish, shark, king mackerel and tilefish for high levels. Not sure but I think 6 oz of low mercury a week, so two servings is what I have seen. Some types of tuna are higher. Salmon and shrimp have been good to eat in everything I have read. I think tilapia is fine for mercury, but not as high in omega3s, so should be okay to eat. I love tilapia too. Mahi Mahi is one of the medium level ones I think. The second website has a small chart about half way down with safe, limited amounts and ones to avoid.
http://www.fda.gov/Food/FoodSafety/Product-SpecificInform...
http://www.pregnancy-info.net/foods_to_avoid.html
I have also seen to avoid a lot of farm raised fish because of other health issues and they contain more PCBs.
Good luck to you!