It's some kind of Threadfin. Some species are freshwater (Paradise Threadfin) but most are saltwater or brackish. There are a few coastal species in the US, the Atlantic Threadfin and the Barbu, which get to about 24", but the Giant Threadfin and the King Threadfin from Africa can get to 5' plus. A lot of people think they are related to Sea Robins but I'm pretty sure their closest relatives are Mullet and Silversides. Awesome fish but tough to keep.
i've been looking for one thats NOT the paradise threadfin. some are fresh some are salt some move in between ... i love the "fourfinger" threadfin or giant threadfin : )