A brackish-water aquarium is an aquarium where the water is brackish (semi-salty). The range of "saltiness" varies greatly, from near freshwater to near marine and is often referred to as specific gravity (SG) or salinity. Brackish water aquaria is a popular specialization within the fishkeeping hobby. Many species of fish traded as freshwater species are actually true brackish species, for example mollies, Florida flagfish, and some cichlids such as chromides and black-chin tilapia. There are also several popular species traded purely as brackish water fish, including monos, scats, archerfish, and various species of pufferfish, goby, flatfish, and gar. Generally, aquarists need to maintain a specific gravity of around 1.005 to 1.010 depending on the species being kept, but practically all brackish water fish tolerate variations in salinity well, and some aquarists maintain that regularly fluctuating the salinity in the aquarium actually keeps the fish healthy and free of parasites.