Also try and take a look at the Aqualog "Brackish Fishes" book, currently the only book out there with any real coverage of these fish. Specifically, it'll help you identify them (a few like the 'prehistoric monster fish' are freshwater, but as CFC says, most need mid brackish to marine).
There are fish that are broadly similar in looks and habit, too. Check out 'freshwater' lionfish, Notesthes robusta, and the waspfish, Tetraroge barbata, both members of the Tetrarogidae. Both these fish need brackish to marine.
None are "easy" fish, and at the very least you will need to start off with a supply of live foods, such as river shrimps, earthworms, or worst case scenario, small fish. As with marine lionfish, most/all can be weaned onto dead foods, but it takes time and patience.
Cheers,
Neale