Reef Salinity: ppt vs Specific Gravity

Two ways to say the same thing

ppt (parts per thousand) describes salinity directly: about 35 grams of dissolved salts in a kilogram of seawater. Specific gravity (SG) compares the water’s density with that of pure water. Saltier water is denser, so SG goes up with salinity.

The catch is that density also changes with temperature. The same water gives a different SG at 20 °C than at 25 °C. A ppt figure doesn’t have that problem, which is why Bulk Reef Supply recommends tracking salinity in ppt.

Calculated with the UNESCO seawater equation, for a reading at 25 °C compared with pure water at 25 °C. That’s the basis aquarium refractometers report on. Use the salinity converter for other temperatures.

Measuring it: refractometer, meter or hydrometer

A refractometer measures how much the water bends light. Most have automatic temperature compensation, so the water’s temperature doesn’t matter. Calibrate it with a seawater standard rather than RODI water. Bulk Reef Supply explains that many hobby refractometers are designed for salt-in-water brine, which reads slightly differently from real seawater, and a seawater standard corrects for that.

A digital salinity meter measures conductivity and reports ppt directly. It needs calibrating too. A swing-arm or floating hydrometer measures density at the water’s current temperature. It’s cheap, but less precise, and it’s calibrated for one temperature, usually printed on it.

Why hydrometer readings mislead

A hydrometer calibrated at 15.6 °C (60 °F) that reads 1.023 in a 26 °C tank isn’t telling you the water is at 30.5 ppt. Allowing for temperature, it’s about 33.6 ppt. That’s a big gap, and it’s why so many tanks run lower than their owners think.

The salinity converter lets you enter the water temperature and the hydrometer’s calibration temperature to get the right figure.

Keeping it steady

Salt doesn’t evaporate; water does. As water evaporates, salinity rises. Top up with fresh RODI water, never saltwater. An automatic top-off (ATO) keeps salinity much steadier than topping up by hand.

When you mix new saltwater for a water change, match it to your tank before it goes in. Red Sea’s mixing guide covers checking salinity in each batch. If salinity has drifted, correct it over a few days using top-off or slightly stronger or weaker change water, not all at once.

Check it often, calibrate monthly

Check salinity weekly, and after any manual top-up or water change. Calibrate your refractometer or meter about once a month; Bulk Reef Supply suggests monthly too. A refractometer that has drifted will happily give you a confident wrong answer.

Is 1.025 OK for a reef tank?

At 25 °C, 1.025 is about 33.2 ppt. It’s a little below natural seawater. Many tanks are fine there, but 34–35 ppt is the usual target.

Should I calibrate my refractometer with RODI water?

Zero it with RODI water if the manual says so, but check it with a 35 ppt seawater standard too. That’s the reading that matters.

How quickly can I change salinity?

Slowly, over several days rather than in one go. Small steps through your top-off or water changes are gentler on livestock.