Salinity

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Salinity

Marine aquaria levels:

1.021 -1.025 SG

27 - 33 PPT

World salinity levels plot.

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Introduction

Water salinity based on dissolved salts in parts per thousand (ppt)
Fresh water	Brackish water	Saline water	Brine
< 0.5	0.5 - 35	35 - 50	> 50

In salt water aquariums salinity levels are maintained at a Specific Gravity of 1.021 -1.025 SG or 27 - 33 PPT (Parts per thousand). An SG of 1.025 is typical level of salinity for a reef system. A Hydrometer or Refractometer should be used to test the SG of the water column. Any adjustments in the salinity level of the water column, should always be done slowly.

Refractometers are the chosen tool to use to check salinity levels by most seriously devoted aquarists because the salinity readings are instantaneous. Hydrometers are less accurate and may take some time to equalize.

Water salinity Levels
Fresh water	Brackish water	Saline water	Brine
< 0.05 %	0.05 - 3 %	3 - 5 %	> 5 %
< 500 ppm	500 - 30 000 ppm	30 000 - 50 000 ppm	> 50 000 ppm

The technical term for saltiness in the ocean is salinity, from the fact that halides—chloride specifically—are the most abundant anions in the mix of dissolved elements. In oceanography, it has been traditional to express salinity not as percent, but as parts per thousand (ppt or ‰), which is approximately grams of salt per liter of solution. Other disciplines use chemical analyses of solutions, and thus salinity is frequently reported in mg/L or ppm (parts per million). Prior to 1978, salinity or halinity was expressed as ‰ usually based on the electrical conductivity ratio of the sample to "Copenhagen water", an artificial sea water manufactured to serve as a world "standard"[1]. In 1978, oceanographers redefined salinity in the Practical Salinity Scale (PSS) as the conductivity ratio of a sea water sample to a standard KCl solution[2][3]. Ratios have no units, so it is not the case that a salinity of 35 exactly equals 35 grams of salt per litre of solution[4].

These seemingly esoteric approaches to measuring and reporting salt concentrations may appear to obscure their practical use; but it must be remembered that salinity is the sum weight of many different elements within a given volume of water. It has always been the case that to get a precise salinity as a concentration and convert this to an amount of substance (sodium chloride, for instance) required knowing much more about the sample and the measurement than just the weight of the solids upon evaporation (one method of determining "salinity"). For example, volume is influenced by water temperature; and the composition of the salts is not a constant (although generally very much the same throughout the world ocean). Saline waters from inland seas can have a composition that differs from that of the ocean. For the latter reason, these waters are termed saline as differentiated from ocean waters, where the term haline applies (although is not universally used).

Salinity is an ecological factor of considerable importance, influencing the types of organisms that live in a body of water. As well, salinity influences the kinds of plants that will grow either in a water body, or on land fed by a water (or by a groundwater). A plant adapted to saline conditions is called a halophyte. Organisms (mostly bacteria) that can live in very salty conditions are classified as extremophiles, halophiles specifically. An organism that can withstand a wide range of salinities is euryhaline.

Salt is difficult to remove from water, and salt content is an important factor in water use (such as potability).

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