Hailea 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) with color-blocked “safe zone” to make routine testing quick and accurate. Features large swing-arm pointer, extra-wide stable base and “easy-dip” design to keep your hands dry while obtaining water sample. 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 We suggest “breaking it in” by filling with saltwater and letting it sit for a few hours, then rinsing well with clean freshwater after each use.