Salinity & Specific Gravity: Measuring Saltwater Correctly
Salinity is the concentration of dissolved salts in water, usually measured in parts per thousand (ppt). Specific gravity (SG) measures the same thing indirectly, comparing the density of tank water to pure fresh water, and it is the number most hobbyist hydrometers and refractometers actually display. Natural seawater sits at about 35 ppt, which works out to roughly 1.026 SG at 25 degrees Celsius, and most reef tanks are run close to that, typically between 1.023 and 1.026 SG.
Salinity vs Specific Gravity: Why Two Numbers for One Thing
Salinity and specific gravity describe the same underlying property of the water, but they get there differently. Salinity is a direct measure of how much salt is dissolved, in parts per thousand. Specific gravity is an indirect measure: it compares the density of your tank water to pure fresh water at a reference temperature, and because dissolved salt makes water denser, a higher SG reading means more salt is present. The two track each other closely enough that hobbyists use the terms interchangeably in practice, but SG is what nearly every testing instrument sold to aquarium keepers actually reads, since density is simpler to measure with a floating hydrometer or a refractometer’s refraction angle than salinity is to measure directly.
How to Measure Specific Gravity Accurately
Temperature has to be controlled before a reading means anything, because specific gravity falls as water gets warmer. Nearly every hydrometer and refractometer on the market is calibrated to read correctly at 25 degrees Celsius (77 degrees Fahrenheit), so a sample pulled straight from a tank running warmer or cooler than that will read wrong even on a perfectly good instrument. With a hydrometer, fill a clean, dry measuring container with your sample, let the hydrometer float freely without touching the sides or bottom, and read the scale level with the water surface itself, not the curved meniscus that creeps higher at the edges. That meniscus is the single most common reading error. Refractometers need their own calibration check against a known reference fluid before each use, since drift is common and a refractometer that’s been sitting unused for months can read off by enough to matter.
Common Salinity Testing Mistakes
The most frequent error is reading temperature-uncorrected water, which throws the number off in a way that looks like a real salinity problem but isn’t one. The second is trusting a cheap swing-arm hydrometer without ever cross-checking it, since these are the least accurate instrument in common use and can drift enough to leave a tank running consistently high or low without the owner noticing. The fix for both is the same: test at the right temperature, and check any hydrometer periodically against a refractometer or a known-accurate reference instrument, since a device that never gets cross-checked can be wrong for months before anyone catches it.
What Salinity Range to Target
Fish-only marine tanks tolerate a wider range than reef tanks, roughly 1.020 to 1.025 SG. Reef tanks with corals and invertebrates are far less forgiving of swings and are usually held between 1.023 and 1.026 SG, close to the 1.026 SG of natural seawater. Whatever number you settle on, consistency matters more than hitting an exact figure. A slow, deliberate drift over weeks is far less stressful to a tank’s inhabitants than a fast swing caused by mismatched top-off water or a bad test reading.
FAQ
What is the specific gravity of salt water?
Natural seawater measures about 1.026 SG at 25 degrees Celsius, corresponding to roughly 35 ppt salinity. Reef aquariums are commonly run between 1.023 and 1.026 SG, close to that natural figure.
What SG should I run my reef tank at?
Most reef keepers target 1.025 to 1.026 SG, close to natural seawater, since corals and invertebrates are the least tolerant of salinity swings. Fish-only tanks have more room to run a bit lower.
Why does temperature affect my salinity reading?
Both hydrometers and refractometers are calibrated to a reference temperature, almost always 25 degrees Celsius. Water expands as it warms, which lowers its density and makes the same amount of dissolved salt read as a lower specific gravity, so a warm or cold sample gives a false reading even on an accurate instrument.
What’s the difference between salinity and specific gravity?
Salinity is a direct measurement of dissolved salt in parts per thousand. Specific gravity is an indirect measurement, comparing your tank water’s density to pure fresh water. They move together and describe the same underlying property, but specific gravity is what almost all hobbyist equipment is built to read.
Is a refractometer more accurate than a hydrometer?
Generally yes, especially compared to a swing-arm hydrometer, which is the least precise option in common use. A well-calibrated refractometer or a precision glass hydrometer is worth the upgrade for anyone keeping corals or invertebrates, where small, undetected swings cause chronic stress.
Shop Salinity Testing
The Tropic Marin High Precision Hydrometer is a precision glass reference instrument that never needs recalibrating, and the wider Marine Aquarium Test Kits range covers salinity testing more broadly. Marine Aquarium Salt & Sand has the salt mixes to hit your target once you know what you’re aiming for.