ive been looking for these and they are brackish. i do think there is a full freshwater species out there but those arent usually sold in pet stores. a helpful hint is that when you see a fish called a "freshwater _________" 99% of the time it is brackish. weird huh? the reason for this is because everyone is familiar with the saltwater species of the fish and dont know what brackish is. also most soles get to atleast 6 inches and need sand not gravel.
there are some fully freshwater bumblebee gobies but usually fish stores tend to be clueless and arent sure where they came from so you need to find that out to be 100% sure.
Both the BBGs and the soles"flounder" are hard to feed so usually many tank mates are not acceptable. i am actually looking for both species but since petco is my only lfs i dont think ill find any any time soon.