It’s high up the food-chain, so it contains all the crap of all the stuff it eats, which tends to mean higher levels of mercury. Mercury suppresses immunity among other nasty stuff. I don’t know how much you’d actually consume from a piece of tuna, but you’re better off sticking to smaller fish lower down the food-chain until you feel better.
From what I’ve heard, heavy metals can be incorporated into candida biofilms, and mercury detox may be one element of die-off. Don’t quote me on that though.