Durable, clear acrylic, swing-arm hydrometer is easy to use and easy to read. Full-range results in both salinity and specific gravity (0 to 40 ppt salinity and 1.000 to 1.030 specific gravity),Quick and accurate. Features large swing-arm pointer, extra-wide stable base. Temperature-corrected for use between 68° and 85°F with specific gravity readings accurate to ± 0.001. Essential equipment for saltwater aquariums.These testing devices are used to maintain the ideal specific gravity (SG) for your marine aquarium fish, corals and other invertebrates.