Marine fish and corals require stable water conditions, which makes regular checking of salinity important. It\'s also necessary to check salinity when making up new water for water changes. The TetraTec Hydrometer makes it simple to accurately check the salt content of water, giving precise readings over a wide temperature range (20 - 29C). It works by measuring the specific gravity (relative density) of the water, and converting this to salinity. All you need to do is fill up the hydrometer with water, and read the value off the scale printed on the side.