In a marine aquarium, salinity refers to the amount of dissolved salts in parts per thousand (ppt). Specific gravity also measures salinity and expresses the density of the aquarium water compared to the density of water. The specific gravity of water is 1 and anything heavier than water has a specific gravity that is above 1. The ocean has a salinity of between 34 to 37 ppt. At 35 ppt, seawater has a specific gravity of 1.026. Aim to maintain a specific gravity of between 1.020 to 1.026 in your marine aquarium. Salinity refractomter is the useful tool.
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