Red Sea Fish Pharm ARE12010 Hydrometer with Digital Thermometer for Aquarium Hydrometer with digital thermometer The most basic water quality parameter in a saltwater aquarium is the salinity The unique, patented design of hydrometer enables the hydrometer to be installed permanently inside the aquarium or sump Giving a continuous real-time reading of S.G. in addition to being used as a regular hydrometer for periodic S.G. measurements This hydrometer also includes a digital thermometer