Verified answer

Columbian sharks, dragon gobies, mollies are from Mexico I believe.... Here's a good link that lists some brackish fish and possible plants that may do ok in a brackish tank, need to research some of those fish though, some are from Asian areas I know:

http://www.thekrib.com/Fish/brackish.html

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I can't come up with a single fish species we see in the trade that is both brackish and from South America.

As for plants for a brackish tank, look for those with thick, hard waxy leaves like some species of Anubias. They are better able to resist the salt.

MM

I'm not sure about the origin, but archer fish, scats and mudskippers are all brackish water fish that will also do well in fresh water. Contrariwise, mollies do well in brackish or even sea water.(if they are any different?)