You know I don't want to piss on the parade but mercury is primarily toxic because it accumulates over time. This is why younger fish contain less.
It is true that with some "chellating" (sp?) or colon-cleansing or whatever you may be able to shift some but there is no such thing as a safe amount of mercury per week. If for example you were to halve the mercury via some brand, a full 50% reduction, that just means it will take a fortnight to receive the same amount as a week of the normal stuff.
I'm not saying don't eat tuna, I eat it myself, my point is just that I find it a little disturbing that there seems to be a consensus of "cut down a bit and mercury's no problem."
My own view is a can a day is going to be well outside the body's ability to remove it, seeing as there is no specific mercury-removing mechanism. Some does get removed, virtually by accident, so small amounts are no problem but unless we're talking something like a 90%+ reduction I really couldn't recommend it on a daily basis.
Bummer I know, it's almost pure protein but there ya go. Sorry.
B.