My intake is nowhere near as much these days, as my training and goals are completely different, but I would regularly have 2 meals a day incorporating tuna, and also an occasional 'tuna shake' (small tin of tuna, half a pint of non fat milk, big dash of tabasco, all blended together
Tuna, milk and eggs were my mainstay of protein for around 12 years, purely on a financial basis. I could not afford whey protein convenience bars, or 'great tasting' fudge brownie casseine shakes. i would get protein shake mix as birthday presents from friends and family, but the rest of the time i liked to keep it old school.
I never experienced any problems, but i am not dismissing what you say. A thousand rocks can make a mountain.
I would say however that if what you experienced was mercury related, then your base level of mercury must have been elevated for the trace amount in the tuna to tip you over the edge, so there may be problems elswhere.
I would suggest a medical exam testing for mercury to be on the safe side.