It looks like a wolf fish or maybe a type of goby (because of the fin after it's dorsal, wolfs don't seem to have those on the ones I have looked at), do not what type because it isn't my thing but that should give you a basis for research.
Does the brackish tank already have other fish in it? If so, you can't just put it back in the wild without possibly cross contaminating the pathogens between captive bred and wild caught fish.