No offense mate, but if you had done your research then you wouldn't be asking about fish like basa etc. Mercury is an accumulator and is therefore only an issue in older and larger fish. Basa (if you had done your research) are harvested within 12 months of age, therefore they do not have enough time to accumulate a mercury level that would be in any way shape or form a worry.
Same with canned tuna and salmon in australia, they are generally smaller fish that are young and fattened up in pens at an accelerated rate, and thus haven't had time to accumulate mercury. Food testing and standards in Australia are some of the highest in the world, I wouldn't be concerned about eating Australian seafood.
But for me, the fact that Basa swim in cages underneath huts which people in vietnam use as a toilet would be a bigger concern for me, plus we have some of the best seafood in the world - I dont understand why anyone would eat imported fish
seems crazy to me.