The Bumblebee Goby (B. xanthozona) is a small, bottom dwelling, carnivorous fish that is distinguished by its solid unbroken black stripes and bright yellow body. Potential owners beware: Many aquarium stores will sell both Brachygobius xanthozona and Brachygobius nunus as Bumblebee Gobies. The problem lies in the fact that B. nunas is a different species altogether and requires brackish water to survive. B. nunas technically is the Gold Banded Goby, but is seldom sold as such. You can easily spot a B. nunas by its broken black stripes and black spots.
If it's indeed a B. nunas, brackish water of 1.004 SG is what you want to keep it in, although I've read that as long as you keep your pH and hardness up, it can be kept in freshwater. Best to try to match the water it's in when you get it. I'd contact the seller to find out.