There is a species of BB Goby that is specifically freshwater. There is a HUGE misconception throughout the internet and fish communities that all BB Gobies are brackish and it's not the case.
Read a few studies where it's shown that most species of BB Gobies do fine in either, although there are a few that are strictly brackish, and a few that are strictly freshwater (but can still last - but not thrive - in either). The local fish store has a fish listed by latin name and its one of the ones that is strictly freshwater.
So, I'm not too concerned. If I notice anything too off, I can always make adjustments (i.e. if I was sold the wrong fish), but this
LFS is pretty spot on.