I read an article last month about a couple women who had mercury poisoning from eating too much fish. It recommended not eating the fish that tend to accumulate mercury (tuna, swordfish, some others) more than twice a week. It doesn't affect you instantly, it builds up over time, and it took their doctors months to finally figure out what the problem was.
I am 100% dead serious in saying that you ought to stop eating that much tuna, because you are headed for nerve problems. And by the time you realize it's causing problems, the damage is done.
Don't take my word for it, do some serious research.