Parameter Reference
Salinityppt / SG
Natural seawater is 35 ppt. Your reef tank should match. The most common salinity error is using an uncalibrated refractometer or a 20-year-old swing-arm hydrometer — both of which read low by 0.001–0.003 SG, leading to under-salted tanks and stressed livestock.
Target ranges
| Beginner-safe | 33–36 ppt |
| Optimal (consensus) | 35 ppt (1.0264 SG @ 25 °C) |
| Triton target | 35 ppt |
| Natural seawater | 34.5–35 ppt (avg) |
| Conductivity at S=35, 25 °C | ~53 mS/cm |
| PSS-78 reference | C(35,15 °C,0 dbar) = 42.914 mS/cm |
Why it matters
Marine fish, corals, and invertebrates evolved at 35 ppt. Their osmotic balance, ion exchange across gills, and skeletal calcium uptake are all tuned to that figure. Drift to 30 ppt and the entire system spends energy on osmoregulation it should be spending on growth. Drift to 38 and the opposite — dehydration stress, depressed dissolved oxygen.
Most salt mixes target slightly elevated levels in the bag (35–36 ppt at "1 cup per gal"). Mix to a known reading and store the empty container — every restock bag has small batch variations.
Symptoms of drift
Too low
Below 33 ppt: osmotic stress on marine organisms, reduced ion concentrations relative to seawater, coral bleaching at sustained low salinity. Acute drops below 28 ppt are lethal to most stony coral within hours.
Too high
Above 37 ppt: dehydration stress, reduced dissolved oxygen carrying capacity, salt creep accumulation on rims and equipment. Acute climbs to 40+ ppt cause coral retraction and fish stress.
Testing
Get a $25 ATC refractometer. Calibrate monthly with 35 ppt standard solution (not RO/DI — that's the most common mistake and it reads 0.001–0.003 SG low). Apex or Trident probes are excellent for continuous monitoring but lag refractometers on absolute accuracy. Discard swing-arm hydrometers, including the Coralife Deep Six — they're consistently off by 0.002+ SG.
Refractometer vs hydrometer — the only debate that still matters
Three instruments measure salinity in the hobby. They are not equivalent.
Swing-arm hydrometer (Coralife Deep Six, SeaTest)
$10 plastic boxes with a needle that swings up as the water lifts it. Universally inaccurate — reads 0.002–0.004 SG low after a few years of use because the pivot collects salt creep. They were standard equipment 20 years ago because nothing better existed at the price point. Today, the consensus on Reef2Reef and from Randy Holmes-Farley is the same: throw it out.
Glass hydrometer (Tropic Marin, ABS)
$30 glass floats with a paper scale. Properly calibrated at 25 °C they read accurately, but they break easily and require correcting for temperature manually. Mostly historical now.
Refractometer (with automatic temperature correction)
The right answer. $20–40 on Amazon for the ATC versions. Light shining through a saltwater sample bends by an angle that depends on salinity; the eyepiece shows where that angle hits a printed scale. ATC (Automatic Temperature Compensation) is essential — non-ATC refractometers shift readings 0.001 SG per 5 °F.
Calibration matters. Calibrate monthly using a 35 ppt salinity calibration solution (Pinpoint, Red Sea, or BRS). The most common mistake is calibrating with RO/DI water at 1.000 SG — the scale's slope is steep at the seawater end, so a zero-point calibration leaves you 0.002–0.003 SG off at 1.026. Use the salinity standard.
Conductivity probe (Apex, Trident, Neptune)
$80–150. Continuous reading, perfect for trend monitoring, but the absolute accuracy depends on monthly probe calibration with two-point standards. Use it for "tell me when salinity drifts" alarms; use a refractometer for absolute readings.
SG, ppt, and conductivity — the conversion shorthand
- 35 ppt = 1.0264 SG at 25 °C = ~53.1 mS/cm
- 33 ppt = 1.0248 SG = ~50.5 mS/cm
- 36 ppt = 1.0272 SG = ~54.4 mS/cm
The salinity converter calculator does the SG ↔ ppt ↔ mS/cm conversions with temperature correction.
FAQ
- Refractometer or conductivity probe?
- Refractometer for spot checks (calibrated monthly), conductivity probe for trend monitoring (alerts on drift). Both, ideally. Don't trust a controller probe alone — they drift over months.
- Why does my SG differ between morning and night?
- Temperature. SG drops as temperature rises. A reading taken at 76 °F before the lights come on reads slightly higher than the same water at 80 °F midday. Most refractometers have ATC; cheap ones don't.
Sources & references
- 01Randy Holmes-Farley — Specific Gravity: Oh How Complicatedhttps://reefs.com/magazine/chemistry-and-the-aquarium-specific-gravity-oh-how-complicated/
- 02Randy Holmes-Farley — Optimal Parametershttps://www.reef2reef.com/threads/optimal-parameters-for-a-coral-reef-aquarium-by-randy-holmes-farley.173563/
- 03Salinometry — PSS-78 referencehttps://salinometry.com/pss-78/
- 04Global Seafood Alliance — Typical seawater characteristicshttps://www.globalseafood.org/advocate/typical-chemical-characteristics-of-full-strength-seawater/
- 05Triton — DIY salt calculatorhttps://www.triton.de/en/toolkit/triton-diy-salt-calculator
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