Is canned tuna good for you or it usually is full of conservants, sodium, mercury, heavy metals and so forth? Is it safe to eat everyday?

Mercury toxic; NO!!! There are good things in tuna but it tends to be high in mercury. Experts advise eating canned albacore no more than 3 times a month, though it is ok to have "light" or skipjack tuna twice a week (but not both!). See http://www.Nrdc.Org/health/effects/mercury/tuna.Asp.