What a specific gravity reading actually means
The hobby quotes 1.005 as though it were a property of the water. It is a property of the water and of how warm it was and of what temperature the instrument was scaled for. This guide explains the three quantities that get called salinity, what each instrument actually measures, and why AquariumHQ's converter refuses to answer certain questions.
Short answer
A hydrometer reading is a comparison, not a measurement of salt: your water against pure water at whatever temperature the instrument was scaled for. Two temperatures are therefore built into every such figure, and one quoted without them cannot be turned back into a salinity by anybody.
Three quantities, one word
One tank at practical salinity 15 reads across a range of about 0.002 specific gravity between 18 °C and 30 °C without anything about the water changing. That spread is wider than the band most brackish keepers are trying to hold, which is why the temperature is not a refinement.
- PRACTICAL SALINITY is what the oceanographic literature means by salinity: a conductivity comparison against a reference potassium chloride solution, written without a unit because it is a ratio, and defined only between 2 and 42.
- PARTS PER THOUSAND, or ‰, counts mass: grams of dissolved salt in a kilogram of water. Close to practical salinity in real seawater, and not the same thing being measured.
- SPECIFIC GRAVITY is what your hydrometer shows: a density ratio whose denominator is pure water at the temperature the scale was built around — 60 °F on most aquarium glassware, 20 °C on laboratory instruments.
Scale: 1.006834526899169 to 1.0130109203775122 specific gravity.
What each instrument is doing
- A glass hydrometer floats at a depth set by density, and its scale was etched for one temperature printed on the stem. It responds to warmth whether you want it to or not.
- A swing-arm hydrometer is the same principle in plastic and the least repeatable instrument commonly sold. One bubble on the arm moves the reading.
- A refractometer measures how much the water bends light, on a scale computed for seawater at around 20 °C. It is least reliable in exactly the dilute range brackish keepers work in.
- A conductivity meter measures the quantity practical salinity is defined from, which makes it the instrument the standard equations actually speak about.
Why the converter sometimes refuses
AquariumHQ implements the two published equations — the 1978 practical salinity scale and the 1980 one-atmosphere equation of state — rather than approximating them, and both are defined over stated ranges. Below practical salinity 2 the salinity scale does not exist, so the site gives you density and specific gravity, which come from an equation defined from zero, and gives no conductivity figure at all.
That floor is lower than it sounds. Practical salinity 2 is about 0.9993 specific gravity at 26 °C, below what any hobby hydrometer resolves; a brackish tank at 1.005 is around practical salinity 9.6 and comfortably inside the scale. What meets the floor is the lightly salted freshwater tank measured on a conductivity meter.
A specific gravity below 1.000 is also not a fault. Warm water measured against a cold calibration temperature is genuinely less dense than that reference, and the ratio goes under one with nothing wrong.
What to do about it
- Record the temperature next to the reading, every time.
- When you quote a specific gravity to somebody, quote the sample temperature and the instrument's calibration with it, or the figure is not reproducible.
- Compare your tank against itself at a consistent temperature rather than against a number from a forum.
- Do not read three decimal places as three decimal places of accuracy on a swing-arm instrument.