canned chunk light tuna is fine. the larger albacore and solid pieces of tuna are from fish that have grown to enormous size, thus consumed a lot more food from lower down on the food chain, thus have had a chance to developed higher Hg levels in thier system, thus higher levels of Hg in the flesh.
basically: the higher up you are on the food chain, the higher the levels of toxins in your body.
since chunk light is made from the smaller ones, you're ok.
i eat 3-4 cans/day.