As far as I knew, they are freshwater species, but you may very well be right in that they may grow into brackish.
That said, to the original poster, I suggest you do some serious research if you'd like to keep the fish. They require some special care and really need a sand substrate with lots of smooth rock and wood. They are also rather delicate to water chemistry, meds, and general parameters.
Also, I see no acceptable reason of why you have not been feeding the poor thing.
Feed it meaty foods like bloodworms, krill pieces, shrimp pellets or other similar foods, asap.
Research before you buy is key.