From The Aquarium Wiki
SG: Specific Gravity
- Specific gravity as it pretains to this hobby is the measure of the density of water. The more salts that are in the water the more dense it is and the more objects will have the tendancy to float in the water.
- There are three popular methods for testing SG in the hobby. First is a hydrometer, this is a device with a scale on it that floats in water and you read the number that is at the surface of the water. Second is a swing-arm device, this is a clear plastic device that is filled with a sample and an arrow of some type points to the SG of the water. The third device is a refractometer, this device has a sample window and an eye glass. When you view through the viewing window the light refracts through a prisim and places a blue line on an easy to read scale.
- The problem with devices one and two are that magnesium alters the results, as well as altitude.
- For brackish water a SG of 1.005 to 1.010 is desired, and for marine water a SG of 1.020 to 1.025 is desired.