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Specific Gravity vs. Salinity

Specific gravity and salinity are both important parameters to monitor in a marine aquarium to maintain a healthy environment for marine organisms.

Specific gravity is a measurement of the density of the water compared to the density of pure water at a specific temperature. It is typically measured using a hydrometer or a refractometer and is expressed as a ratio or a unitless number. In a marine aquarium, the specific gravity of the water is affected by both the amount of dissolved salt in the water and the temperature of the water. The specific gravity of a salt solution will rise as the temperature declines.

Salinity, on the other hand, is a measurement of the concentration of dissolved salts in the water. It is usually expressed in parts per thousand (ppt) or as a percentage. In a marine aquarium, the salinity level can be maintained by adding salt mix to the water. The measurement of salinity is not affected by temperature.

Marine aquarists traditionally used specific gravity to measure the concentration of salt primarily because of the ease of use and cost-effectiveness of hydrometers.

Maintaining proper levels of specific gravity and salinity is essential for the health and well-being of marine organisms. A specific gravity range of 1.020 to 1.026 is considered suitable for most marine fish and invertebrates, while a salinity level of 35 ppt is typically recommended. However, it is important to check the specific requirements of the particular species of fish and invertebrates in the aquarium and adjust the specific gravity and salinity levels accordingly.

It is also important to note that changes in specific gravity and salinity can have a significant impact on the health of marine organisms, and sudden changes in these parameters should be avoided. Regular monitoring and gradual adjustments are key to maintaining a stable and healthy marine aquarium environment.