Tuna

Few questions about Tuna for anyone that knows...I was reading in a book that states that Tuna contains the good fish oils that we need to consume, and that it is not one of the high mercury content fish.  So, the questions are:

1.  Do you consider the above statement about oils and mercury to be accurate?

2.  Is canned tuna OK(or more specifically, does canned tuna in water still contain the appropriate oils I'd be seeking)? And the kind I get is chunk light. (I'm hoping canned tuna is ok, cuz buying fresh tuna fish would be expensive and I don't know how to prepare it).

3.   as far as mercury; is canned tuna OK to consume on a daily basis? 

4.  What kind of things do you add to it?   Used to have chopped onion, might do that again...

i remeber something about its good to have tuna 3 days a week, but shouldnt have it more then that. so i am not sure. maybe someone else will know more.

as for tuna salad here is how i make it, 1 can tuna mayo, spicy mustard, celery salt, pepper, sweet relish & hard boild eggs. can add onoins as i am allgic to em, but my family adds it on there own.