This is a common thing in regards to knight gobies.
It really depends on your LFS and who they got the gobies from. Knight gobies are mostly wild caught that you get in the stores, but there are some that have been bred successfully.
The ones that come straight from the wild are given a 50/50 chance of getting them properly acclimated to either fresh or brackish water. The reason for this is because it all depends on where they are caught in regards to the rivers. If they are caught more inland (freshwater areas) then they will be happier in freshwater whereas if they are closer to the ocean/brackish areas they will be happier with crackish conditions.
If your LFS happens to sell some that are captive bred, then you are most likely (about 93%) to best go with freshwater as that would be the most common for breeders to get the most $$$ for thier product.
In all honesty, ask your LFS if they know if they are Wild caught or captive bred. If they don't know then the best of luck to you. I would go with freshwater over brackish as it seems to be the majority there.
