Aquarium Information Source

Specific Gravity and the Hydrometer

Specific gravity is a measurement used to determine the amount of salt that exists in salt water. It is measured using a hydrometer ( pictured below.) When a hydrometer is filled with water, the arm will raise to the appropriate ppm (parts per million,) therefore telling you what the specific gravity of the water is.

In the marine aquarium a specific gravity of 1.021 - 1.024 is considered ideal. Generally, when water evaporates from the aquarium, specific gravity will rise. When specific gets too high, add fresh water; and when it gets too low, add more salt.

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