Hi CJ,
It is not even a bichir. I'm still trying to track down more informations of eels and gobies to identify your fish.
Pls be aware that without info, you may end up with large fish which you may not be able to keep for a long time.
Some may be brackish, some don't. And brackish water fish tend to demand salt as they mature even if they thrive in freshwater as juveniles.