Can't remove mercury
Mercury increases throughout the food chain and is in largest concentrations in carnivorous fish.
('bioaccumulation') the mercury is in the flesh of the fish and cannot be removed through any process. It is best to avoid consumption of certain species known to have high levels of mercury. See http://water.Epa.Gov/scitech/swguidance/fishshellfish/outreach/advice_index.Cfm for more details.