Technically, brackish encompasses the entire realm between freshwater (1.000 specific gravity [SG]) and saltwater (1.021-1.025 SG). Some brackish species, such as bumblebee gobies, silver and New Guinea dats, and archers, prefer the lower end of the spectrum, around 1.005 SG. Other species, such as adult scats, monos, "FW" morays, and green spotted puffers, prefer the higher end of the spectrum, 1.015 or so. As a pratical matter, it may be prudent to keep your brackish water fish near the lower end of the SG spectrum, say 1.005 to 1.010 SG, because most LFS keep their brackishwater fish either in FW or in low salinity, typically 1.005 or less.