1.do not buy anything larger than salmon or tuna.
2.do not buy anything farm raised.
3.do not buy Atlantic Salmon since it has been extinct in the wildness. Atlantic Salmon is farm raised.
4.do not buy fish caught near newly-industrialized countries, including China, Vietnam, Indonesia, South America. Especially we have imported a lot from South America, the economies of which are dominated by mining industry.
5.use shrimp or crab instead since mercury in those creatures is zero.