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Hydrometers Measure The Salinity In Marine & Reef Aquariums.

Marine Aquarium Hydrometers are designed to measure the specific gravity (salt content or salinity) of the water in your marine or reef aquarium tank. The Hydrometer will have been calibrated to take this measurement at a given temperature, normally about 25C.

The salinity of your aquarium will change over time as water evaporates so it is important to regularly check the water with a marine hydrometer to keep the salinity within an acceptable range.