A Hydrometer and a Refractometer are instruments are both used to measure the level of salinity or specific gravity in your aquarium water. Salinity is the relative proportion of salt in a solution. In this case, the amount of salt mix dissolved in your aquarium water.  Specific gravity is the density of a substance relative to the density of water; salt mix to aquarium water. It is very important to maintain a salinity level within a certain range of, 1.020-1.023, for a fish-only tank setup and 1.023-1.025 for a reef tank setup, otherwise the fluctuation will cause your animals to be stressed and you can wind up with a possible end result of a casualty. Both of these instruments are easy to use & we carry a variety with different features to choose from.