The Instant Ocean Testing Hydrometer for Aquariums makes the difficult task of reaching the correct salt level in your marine aquarium much easier. Maintaining the right level is vital to creating and maintaining a thriving marine environment. It’s important to make sure that synthetic sea salt is mixed in the proper ratio to mimic natural sea water. The Instant Ocean Testing Hydrometer is a precise， reliable tool that provides full-range salinity and specific gravity readings， and it’s accurate to ± 0.001. It’s simple to use and read with a clearly marked safe zone.