Salinity: ppt, specific gravity and mS/cm compared
Updated on August 26, 2026 · ~6 min read
Reef salinity is held around 35 ppt, which corresponds to a specific gravity of 1.025-1.026 and a conductivity of about 53 mS/cm. The three units measure the same thing in different ways: the conversion depends on temperature, so always measure at tank temperature.
Salinity is the simplest parameter in a marine tank and the one read in three different units, with three different instruments, calibrated in different ways. That is where the confusion comes from: two aquarists quoting opposite numbers are often measuring the same water.
The three units
ppt — parts per thousand. The grams of salt per kilogram of water. It is the most direct unit and the canonical one: 35 ppt means roughly 35 g of salt per litre. You read it on a refractometer with a ppt scale, or you derive it from the other two.
SG — specific gravity. The ratio between the density of salt water and that of pure water. It is the "1.025" type of number you read on hydrometers and refractometers. ⚠️ It depends on temperature, so the same sample gives two numbers at 20 °C (68 °F) and at 26 °C (79 °F).
mS/cm — conductivity. How well the water conducts current, which depends on the dissolved ions. It is the unit of electronic probes and controllers. It also depends on temperature, and good instruments compensate for it themselves.
The conversion table
| ppt | Specific gravity (SG) | Conductivity |
|---|---|---|
| 28.0 | 1.020 | ~42 mS/cm |
| 30.8 | 1.022 | ~47 mS/cm |
| 33.6 | 1.024 | ~51 mS/cm |
| 35.0 | 1.025 | ~53 mS/cm |
| 36.4 | 1.026 | ~55 mS/cm |
⚠️ These are approximate conversions, and should be taken as such. The relationship between the three quantities is not exactly linear and depends on temperature, on the composition of the salt and on the instrument. The relations used here — the same ones the app applies so as not to give two different results from the same value — are ppt = (SG − 1) × 1400 and ppt = mS/cm × 35 / 53. They are enough to tell whether you are in the right place; they are not enough to compare two decimals across different instruments.
Which value to hold
| Tank | Salinity |
|---|---|
| Reef | ~35 ppt (1.025-1.026) |
| Fish-only marine | 30-35 ppt |
| Brackish | 5-15 ppt |
| Hospital tank / hyposalinity treatment | 16-18 ppt, only for the duration of the treatment |
A reef's reference value is that of tropical sea water, and that is no accident: corals and most invertebrates are adapted to it. A fish-only tank can sit lower without problems — some people deliberately keep it at 30-32 ppt, because it costs the fish less energy — but with corals and invertebrates there is no margin.
And as always in marine tanks: constancy matters more than the value. A steady 34 ppt is better than a 35 that swings.
How to prepare the water
You dissolve synthetic aquarium salt in RO water, not tap water: nitrate, phosphate and silicate from the tap are the most common cause of algae in a reef's first months.
To bring RO water to 35 ppt takes about 36 g of salt per litre — that is roughly 3.6 kg per 100 litres (26 gal). In practice: dissolve slightly less than calculated, wait until it is completely dissolved (the water goes clear, not milky), measure, and adjust. Always in a separate bucket, with a pump moving the water, never by pouring salt into the tank.
Evaporation: the thing that confuses everyone
The water evaporates, the salt stays. So in a tank losing three litres a week the salinity rises, and top-ups are done with pure RO water, with no salt. Topping up with salt water is the most common way of pushing salinity up month after month without understanding why.
A water change is a different thing: there you take salt water out and put salt water of the same salinity in.
In summer, with a fan running over the surface, evaporation can reach several litres a day: it is the period in which salinity moves fastest, and in which an auto top-off system (or a daily reminder) makes the difference between a stable tank and one that drifts.
How to measure it, and with which instrument
Refractometer — this is the right instrument. It costs little, has no parts that foul, and reads well. ⚠️ It must be calibrated, with distilled or RO water: put a drop on, check that the line falls on 1.000 / 0 ppt, and if it does not, adjust the screw. Redo it every few months, and always before believing a strange reading. And use it at room temperature, letting the drop equilibrate for a few seconds before reading.
Conductivity probe — precise and suited to a controller, it must be calibrated with reference solutions and its tip degrades over time.
Swing-arm hydrometer — the least reliable instrument on the bench: bubbles on the arm and salt deposits shift the reading considerably, and it does not compensate for temperature. Fine for telling whether you are in the right ballpark, not for deciding.
Salinity is not corrected in a hurry. You adjust it over several days: RO water to lower it, more concentrated salt water to raise it, always a little at a time. A fast jump is one of the worst stresses for invertebrates and corals — much worse than a value slightly off held steady — because they lack the osmotic regulation fish have.
Brackish, which is a case of its own
A brackish tank sits between 5 and 15 ppt, that is between a specific gravity of about 1.004 and 1.011. Those are values where marine refractometers read poorly (the scale is calibrated for the top end) and swing-arm hydrometers are useless: you need a refractometer with a low-range scale, or a probe.
And salinity must be set to its final value from the start: filter bacteria adapt to the environment they grow in, so changing it after the tank has matured means reopening the cycle.
How Gurglee does it
Gurglee converts between ppt, specific gravity and mS/cm by itself: you write the reading in the unit of whichever instrument you have in hand ("sg 1.026 in the reef", "conductivity 53.5") and the app works out which unit it is from the order of magnitude — 1.0xx is a specific gravity, 35 is ppt — then stores it converted, so two different instruments end up on the same chart instead of two. The reference band follows the type of tank: ~35 ppt in a reef, wider in a fish-only marine, 5-15 ppt in a brackish one.
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