I have 3 of these guys in with my puffers. They are very entertaining, always swimming around chasing each other. I have the freshwater type, as my puffs are 100% FW. I guess I was lucky to find them. Good Luck with yours.
If you want to keep fish that need mild brackish conditions then marine salt added at 2 tablespoons per 10 US gallons (40 litres) of water is sufficient.
There are two species known as bumble bee gobies one of which is found in both FW and brackish and one found in brackish only. Unfortunately the two are barely distinguishable from each other as the only differences is Brachygobius doriae (the brackish only species) has broken dark bands in between solid dark bands and Hypogymnogobius xanthozona (which can go in both FW and brackish) has just solid dark bands.
I have 8 bumblebee gobies - 4 each in 2 tanks...
Where did you buy them from? Next time try Tropical Fish Centre on Hyde Road, Denton - I think they are £1.10 each