Measuring Salinity in a Saltwater Aquarium

Salinity in a saltwater tank isn’t a value you set once and then forget about. It changes every day, all on its own, and the tank’s inhabitants notice it before you do. Corals and invertebrates are sensitive to fluctuations, while fish are more forgiving. This article will show you what the right level is for your tank, why it fluctuates, and how to measure it without having to guess. The basics about the measurement itself—that is, how it differs from conductivity and TDS—can be found on the salinity page.

Guidelines for Saltwater Tanks

Natural seawater has a salinity of about 35 ppt, which corresponds to 3.5 percent or 35 grams of salt per kilogram of water. For aquariums, the following applies depending on the stocking:

Basin	Salinity	Density at 25 °C
Reef tank with corals and invertebrates	34 to 35 ppt	1.0257 to 1.0264
Fish tank without invertebrates	32 to 34 ppt	1.0242 to 1.0257
Quarantine with hyposalinity, temporary, and never with invertebrates	16 to 17 ppt	approximately 1.0092 to 1.0102
Natural seawater as a reference	approximately 35 ppt	1.0264

More important than the exact target value is that it remains stable. A tank that consistently maintains 33 ppt is better off than one that fluctuates between 33 and 36 ppt.

ppt, density, and percentage: which unit applies

In saltwater aquariums, three different terms are used to describe the same thing. ppt stands for parts per thousand—that is, grams of salt per kilogram of water—and is the most direct measurement. PSU is the unit used in oceanography and, in practice, corresponds to the ppt value. Density, often expressed as specific gravity, is, by contrast, a derived value and depends on temperature.

That is precisely where the most common source of error lies: 1.026 at 25 °C is not the same as 1.026 at 20 °C. If you’re using density values from a forum or a salt package, always check what reference temperature they’re based on. A measurement given directly in ppt avoids this problem.

Why the salinity fluctuates in the first place

Salt doesn’t evaporate, but water does. Every liter that evaporates from the tank leaves the salt behind, thereby raising the salinity. In an open tank with a skimmer and circulation pumps, this can easily amount to several liters a day in the summer. Therefore:

• Evaporated water should be replenished exclusively with freshwater—ideally reverse-osmosis water—and never with pre-mixed saltwater
• Replacement water must first be adjusted to the same salinity as the tank and thoroughly mixed; otherwise, the salinity level will fluctuate with every water change
• Automatic top-off systems keep the salinity level very stable, but they can also mask problems—such as an empty reservoir—if something goes wrong
• After adding salt, it takes time and water flow for the salinity level to even out throughout the entire tank

When and how often to measure

For a well-established tank, one regular weekly test is sufficient, plus one before and after each water change. You should also test in the following cases:

• after prolonged heat waves, or when lighting and equipment generate more heat than usual
• if the automatic refill system has failed or the reservoir has run dry
• during the first few weeks of a newly set-up tank, where monitoring should be more frequent
• before introducing new animals, so that the adjustment begins from the correct baseline

What to use for measurement

Hydrometer, Refractometer, or Measuring Device

The hydrometer, or floating spindle, is the cheapest option and the least reliable. Air bubbles clinging to the spindle, salt deposits, and the pool temperature can skew the reading; deviations of one to two ppt are not uncommon in everyday use. A refractometer provides significantly more accurate measurements, but it must be regularly calibrated with a reference solution, and you have to read an optical scale against the light—which, especially in borderline cases, leaves room for interpretation.

An electronic meter displays the value directly as a number in ppt, automatically compensates for temperature, and provides reproducible results because nothing needs to be read or estimated.

The Right Device

The key factor is the measurement range. Pocket Testers like the Salt20 or the EC60 have a maximum reading of 10.0 ppt and are therefore suitable for freshwater, brackish water, and saltwater pools; however, they are not sufficient for seawater with a salinity of 33 to 35 ppt. A suitable option is the 5052 Salinity meter for seawater with a measurement range of 0 to 100.0 ppt, a resolution of 0.01 or 0.1 ppt, and an accuracy of ±1.5% of the full-scale value. Temperature compensation is automatic; calibration is performed once a month at a single point, and the calibration solution is included in the set. In addition to reef tanks, this range also covers hyposalinity in quarantine tanks and aquaculture. An overview of all devices, including those for smaller measurement ranges, can be found in the category Salinity meters.

How to Measure Correctly

• Take the measurement in the tank itself, not in the sump directly below the refill system’s inlet, where the water is temporarily diluted
• Hold the electrode still and wait until the reading stabilizes, rather than noting the first reading
• Rinse the electrode with fresh water after each measurement; otherwise, salt will crystallize on it
• Always take measurements at the same time of day, preferably before refilling; this ensures that the readings are comparable over weeks

Adjusting the Salinity

If the level is too high, balance the difference with freshwater—not by performing a large water change all at once. If it’s too low, completely dissolve salt in a separate bucket of tank water and add the solution slowly; never add dry salt directly to the tank. In both cases, the rule is: go slowly. For corals and invertebrates, a change of more than about one ppt per day is stressful, and stress is exactly what you want to avoid by taking these measurements.

Conclusion

33 to 35 ppt—in a reef tank, more like 34 to 35—and, above all, consistent: that’s essentially all there is to it. The challenge isn’t in achieving the target value, but in keeping evaporation under control and measuring it regularly, rather than being caught off guard once a quarter. You can find out how salinity relates to the other water parameters on the aquarium keeping page, and for the freshwater tank next door, the pH value in the aquarium is a good place to start. If you’re familiar with the warning signs, you can recognize critical situations before the fish visibly suffer.

Frequently Asked Questions