I eat it twice a week, if you've got a healthy lifestyle eat lots of anti oxidants and drink plenty of water it really shouldn't be too much of an issue, but it is not a good thing to have in excess and I've met a couple of bodybuilders who had to have some consultancy from doctors for the mercury in their body. Heavy metals in the body aren't good, but they're all around us, we breathe them in, we eat them we can't really get away from them. There are ways of getting rid of heavy metals, look it up as it will make more sense to you as you know your body better than anyone else.
Back to the point at hand, unless you have health issues and aren't pregnant there's nothing wrong with tuna and other big predatory fish - however, the key is variation and a balanced diet - sure they're great for low fat high protein, but so are other smalled predatory fish.
Think about it this way, the smaller the fish the smaller the amount of toxins, the big fish eats the little fish, which is eaten by a bigger one, then a bigger one eats that one etc... the concentration builds up. Unlike land animals the food chain is usually herbivorous - in the sea there is very much a food chain.
Don't be too excessive on your paranoia it doesn't make sense, everything in moderation. Though a point to note is that toxicity levels have risen within big fish in coastal areas - owing either to natural factors, human factors or just general increase in waste etc...
Ultimately you won't do yourself any harm, but I wouldn't rely on it for daily part of your diet.