Take this while you are eating your tuna.
Arizona Natural Products Calcium Disodium EDTA -- 500 mg - 100 Capsules - Vitacost
It will completely bind the mercury in the tuna. Take it about midway through your tuna meal. It will work best if you are not mixing the tuna with other stuff like pasta, i.e tuna salad.
EDTA is not a "natural" or herbal product. It's an industrial product. It will bind minerals in your gut like a motherfucker so go easy with it.