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Salinity, specific gravity and brackish water

Brackish water is not a single thing. The estuarine species this catalogue holds come from water that swings through a tidal cycle, and the figure a keeper reads off a hydrometer depends as much on how warm the sample was as on how salty it is. This record covers what the numbers mean, which instrument measures what, and where the standard equations stop being defined.

In one sentence

Salinity is how much salt is dissolved in the water; specific gravity is a ratio of two densities at two temperatures and is how the hobby usually measures it. The second number is meaningless unless you know both of those temperatures, and almost nobody quotes them.

Three different quantities, routinely confused

The hobby's habitual figure — 1.005 for a low brackish tank — is a specific gravity, and it corresponds to a different salinity at 22 °C than at 28 °C. On a 60/60 °F hydrometer the same water reads roughly 0.0025 lower at 28 °C than at 15.5 °C, which is more than the width of the band most brackish keepers are aiming at.

  • PRACTICAL SALINITY is defined by a ratio of electrical conductivities against a standard potassium chloride solution. It has no unit — a figure of 15 is written as 15, not 15 ppt — and its defining scale, PSS-78, runs from 2 to 42.
  • PARTS PER THOUSAND (ppt or ‰) is a mass fraction: grams of dissolved salt per kilogram of water. It is numerically close to practical salinity in genuine seawater and it is not the same quantity, because two waters with the same mass of salt and different ions do not conduct alike.
  • SPECIFIC GRAVITY is the density of your water divided by the density of pure water at a stated reference temperature. It is a ratio between two temperatures, and quoting it without them is like quoting a hardness without saying whether it is dGH or ppm.
The freshwater to marine salinity continuum, with aquarium landmarksA horizontal bar from 0 to 40 parts per thousand, divided into the standard salinity classes: fresh below 0.5, oligohaline to 5, mesohaline to 18, polyhaline to 30 and marine above 30. Aquarium landmarks are marked beneath: most so-called brackish aquarium fish are kept between 1 and 15 parts per thousand, a juvenile green spotted puffer sits near the lower end and an adult near full seawater, and the estuarine habitat class as a whole spans 1 to 20.FreshOligohalineMesohalinePolyhalineMarine0510203040Salinity, parts per thousandEstuarine habitat class, 1–20Where most brackish aquarium fish are keptJuvenileAdultSeveral estuarine species move up this scale with age. A green spotted puffer bought as ajuvenile and kept in fresh water is not merely at the wrong salinity — it is at the wrongsalinity for a life stage it left years ago.
Salinity is a continuum and the trade sells it as three categories. 'Brackish' spans a fortyfold range, and a species that needs the top of it will not survive at the bottom. Source: Venice system salinity classes; AquariumHQ habitat class envelopes for the aquarium landmarks.

What each instrument actually does

  • A GLASS HYDROMETER floats at a depth set by the water's density, and its scale was etched for one reference temperature — usually 60 °F, printed on the stem. It measures density, not salt, so it responds to temperature whether you want it to or not.
  • A SWING-ARM HYDROMETER is the same principle in plastic, with a pointer instead of a float. It is the least repeatable instrument commonly sold: a single bubble on the arm shifts the reading, and salt creep changes the arm's mass over months.
  • A REFRACTOMETER measures how much the water bends light. Aquarium models carry a scale computed for seawater at around 20 °C, with automatic temperature compensation built for that scale, so they are least reliable in the dilute brackish range this catalogue is concerned with.
  • A CONDUCTIVITY METER measures the thing practical salinity is defined from, which makes it the instrument the standard equations actually speak about — but it reads total ionic content, so hard fresh water and dilute brackish water can read alike.
60/60 °F (15.56 °C)same water, 18 °C to 30 °C
1.0078 – 1.0110
20/20 °Csame water, 18 °C to 30 °C
1.0086 – 1.0118
25/25 °Csame water, 18 °C to 30 °C
1.0098 – 1.0130
Against water at its maximum density (3.98 °C)same water, 18 °C to 30 °C
1.0068 – 1.0100

Scale: 1.006834526899169 to 1.0130109203775122 specific gravity.

One tank at practical salinity 15, measured between 18 °C and 30 °C. Each bar is the range of specific gravities that one unchanging water gives on an instrument with that calibration. A figure quoted without both temperatures is somewhere inside one of these bars and there is no way to tell where. Source: AquariumHQ implementation of the one-atmosphere equation of state (Millero & Poisson 1981), computed at render.

Where the standard equations stop

PSS-78 is defined between practical salinity 2 and 42. That floor is lower than it sounds — at 26 °C it corresponds to a specific gravity of about 0.9993, below anything a hobby hydrometer resolves, and a brackish tank at 1.005 is around practical salinity 9.6 and well inside the scale. What meets the floor is the lightly salted tank measured on a conductivity meter. Below it the polynomial will still return a number, and that number is not a practical salinity; AquariumHQ's converter refuses the conversion rather than performing it, and says why on the page.

A specific gravity below 1.000 is not a fault either. The reading is a ratio against pure water at the instrument's calibration temperature, so warm brackish water measured against a cold reference is genuinely less dense than that reference and the ratio drops under one.

The density side is better behaved: the one-atmosphere equation of state is stated valid from salinity 0, so specific gravity and density remain available all the way down to fresh water. This is why the converter can still answer part of the question below 2 and not the rest of it.

What this means for keeping brackish fish

Measure at a consistent temperature, or record the temperature alongside the reading — otherwise you are comparing your tank against itself on a moving scale. If you quote a specific gravity to another keeper, quote the sample temperature and the instrument's calibration with it.

A tidal estuary is not a stable environment, and several species in this catalogue meet a range of salinities through a day rather than one figure. That is a reason to be unimpressed by three decimal places, not a reason to be careless: the range an animal tolerates is a different claim from the range it does well in, and this catalogue records them separately where it has evidence for both.

Typical aquarium range

Typical
0–18 practical salinity