Dr.Jeff
Here's a good calculator from the Environmental Working Group for helping to determine tuna consumption -
http://www.ewg.org/tunacalculator
The best form of tuna is Chunk Lite tuna. These are small tuna that don't build up large stores of heavy metals or chemicals. Tuna steaks can be a greater source of mercury, as these come from large tuna.
Mercury doesn't directly affect candida, and if anything, it is candida that affects mercury as it will take it into the cell and biotransform it to a form that is problematic -
http://www.pubmedcentral.nih.gov/pagerender.fcgi?artid=182692&pageindex=1.
Mercury can have a suppressive effect on the immune system and this can play a role in allowing fungal candida to become a problem. Many people are under the misconception that you can't get rid of candida with mercury present. This would only be true if mercury was present in such large quantity that it would suppress your immune system systemically. I have never seen this to be the case. Mercury is more likely to store in tissues and affect the health of the nervous system than anything else.