Brachygobius xanthomelas is the most freshwater tolerant species of Bumblebee goby, and also one of the smallest. While they can live in full freshwater, hard water (like our tap water in the city) is preferred, and they certainly wouldnt mind the addition of a bit aquarium salt either. Bumblebee gobies are very peaceful with other species and your only concern for tankmates should be that you dont keep them with anything big enough to eat them, as this species hardly reaches an inch.