1 . To measure the salinity or specific gravity of the water2 . To make weekly replacement seawater3 . To monitor the aquarium waterSometimes water in the aquarium slowly evaporates leaving salt behind, gradually increasing the salinity of the aquarium. You’ll need to use the hydrometer to monitor this activity so that filtered freshwater can be added to keep salinity constant.Please note salinity should be kept constant at 26 parts per thousand (PPT), 1.019 Specific Gravity for fish only, or 32 PPT/1.023 for fish and corals.