Salmon is one of the best - one of the lowest in mercury and one of the best in Omega-3's. A few serves a week of salmon are fine, in fact you could probably have a serve a day if you really wanted to.

Fresh/Frozen tuna you should avoid. Canned tuna is ok but don't have more than 200-300g a week. Don't eat shark/flake/swordfish etc.

Mercury has a half life in the body of about 70 days, which is why the FSANZ article above mentions for 6 months before ttc. Ie, if you eat a high mercury fish now, half of the mercury will still be in your blood in 70 days, so its more a cumulative thing of how much you take in over a long period.