Sera Marin Floating Glass Hydrometer
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For the uncomplicated monitoring of marine water density
The salinity (salt contents) can be deduced from the measured density.
Since the salinity of marine water in an aquarium must be kept constant, the aquarium water should regularly be checked accordingly. The water to be tested is filled into a sufficiently tall water glass, and the scale of the immersed hydrometer is read off on according to its immersion depth.