the mercury in fish doesn't come from the canning process, it comes from industrial pollutants. The determining factor for your risk in getting heavy metals from your seafood is a matter of where your seafood comes from and where what it ate comes from. Tuna is a fish that eats smaller fish, so not only do you have to think about where the tuna live, but where do its food sources live, and where do the food sources of what those ate come from?
Heavy metals are at really low levels in seawater (unless you're right by a factory or something), but most organisms will absorb very small amounts over time and the process of eliminating heavy metals is very slow, so it accumulates. Not a big deal. The problem is that say you have a shrimp floating around in the water. He's got a tiny bit of mercury in him from the seawater and algae and all that. Along comes a pollock and he eats about a hundred of those shrimp, so he has 100x that level. Along comes a tuna, who eats a few pollock, so he multiplies that amount even more. He and a few of his tuna buddies get eaten by a shark who then gets caught and bought on sale at the fish market.
Back to tuna, the FDA recommends that you not eat more than 2 meals of tuna a week. Is an extra can or two going to hurt you? Probably not, and not right away, just like the fish, it takes time for a person to accumulate a toxic level of mercury, but it would probably be beneficial to replace some of those 6 cans a week with something else like the frozen chicken breast that's always on sale
Spiderpig: spanish mackerel has about half the ppm of mercury on average that albacore tuna has, king mackerel (gulf of mexico, pretty big fish) has nearly double that of tuna, and chub mackerel (the kind you eat out of tins) has around 1/5 the amount that albacore has (depending on atlantic or pacific), as it's a small fish
