SODIAL(R) Aquarium Glass Float Hydrometer With Thermometer Salt Marine 20cm Measure both Temperature and Specific Gravity of the water Green indicators are ideal range of water temperature and specific gravity for most salt water / marine aquariums Instruction:Place the hydrometer into water, and let it flow freely in the water If water level below the green indicator means more fresh water is required If water level above the green indicator means more salt water is required