Why stable salinity matters
Reef animals live in water that sets the conditions for the exchange of water and ions across tissues. A change in salinity changes those conditions. Fish regulate their internal water and ion balance, while many marine invertebrates have internal fluids that track the surrounding seawater more closely. The effects of a change depend on the animal, its acclimation history, the size of the change and how quickly it occurs. This is why a stable, appropriate salinity is more informative than a single reading recorded after an adjustment.
Salinity also changes how you interpret other tests. Evaporation tends to concentrate dissolved constituents that remain in the water, though biological uptake, dosing and precipitation can change individual concentrations independently. If several parameters move together after the sump level falls, check salinity and the top-off system before attributing every change to a separate dosing problem.
What the salinity reading actually means
In aquarium use, 35 ppt is a common reference near typical ocean-water salinity. It helps to know what your instrument is reporting. A refractometer infers salinity from how light bends. A conductivity meter infers it from electrical conductivity. A hydrometer reads density relative to a reference. Specific gravity is a ratio, and its interpretation depends on the temperature convention and the instrument. In formal oceanography, Practical Salinity is derived from conductivity and Absolute Salinity is expressed in grams per kilogram. The scales are related and have distinct definitions. [2] [3]
Worked example: a 300-litre reef with an empty top-off reservoir
Take a reef aquarium with approximately 300 L of circulating seawater, including the sump, at 35.0 ppt. The automatic top-off reservoir runs dry. At an illustrative evaporation rate of 2 L per day, the system loses 6 L over three days. Assume the return pump is still circulating water and there have been no spills, water changes or other significant salt losses.
| Stage | Circulating water volume | Estimated salinity |
|---|---|---|
| Before evaporation | 300 L | 35.0 ppt |
| After three days without top-off | 294 L | 35.7 ppt |
| After replacing evaporation with 6 L of freshwater | 300 L | 35.0 ppt |
With the salt still in the system, a practical estimate is 35.0 × 300 ÷ 294 ≈ 35.7 ppt. The volume-ratio method approximates the seawater mass ratio and ignores small density and volume changes. The values in the table are estimates, so verify the actual salinity with a calibrated instrument. Volumes on the aquarium label can overstate the water in operation because rock, sand and the sump operating level change the amount of circulating water. [2] [3]
Adding 6 L of freshwater restores the original volume and, under the example’s assumptions, the original salinity. Adding 6 L of saltwater at 35 ppt restores the level and introduces extra salt. The resulting salinity would still be about 35.7 ppt. With a top-off system mistakenly supplied with saltwater, that extra salt would accumulate over repeated cycles. Your own evaporation rate will vary with airflow, humidity, temperature and exposed surface area.
Decide what to replace by identifying what left
- Evaporation: replace the lost freshwater, typically with reverse-osmosis deionized water (RO/DI) in a reef system.
- Water change: replace removed seawater with prepared saltwater, checking its salinity and temperature before use.
- Wet skimming, spills or a leak: these can physically remove saltwater. An automatic top-off can replace that lost volume with freshwater and gradually lower salinity. Inspect the equipment and confirm the salinity before choosing a correction.
- Water level rises while salinity falls: inspect the freshwater top-off sensor, reservoir plumbing and potential siphoning. Confirm the measurement before making another water addition.
In a sump system with fixed baffles, the return-pump chamber often shows the falling level first while the display level appears steady. Mark the normal level with the return pump running, and compare readings in that same operating state. Top-off during maintenance can confuse the diagnosis if water is temporarily removed or displaced.
Measure before you adjust
- Check the instrument near the working point. Use a reference compatible with your instrument and intended to represent seawater near the target reading. A pure-water check at zero may miss errors that show up near reef salinity. Follow the instrument’s calibration and temperature instructions. [3]
- Use a fresh sample and a clean prism. Residual salt or freshwater can affect a small sample. Repeat an unexpected result and write down the unit, measurement time and recent maintenance.
- Compare the reading with the water level and log. A low sump and higher salinity are consistent with evaporation. A low sump and unchanged or lower salinity suggest that other processes or measurement error merit investigation.
- Track several readings. Note reservoir refills, top-off interruptions, water changes and wet skimming. If a verified reading is far from your usual target, determine what is happening and evaluate a measured correction suitable for the animals in the system.
Sources
- NOAA JetStream, Sea Water. Environmental factors that raise or lower seawater salinity.
- IOC, SCOR and IAPSO, Thermodynamic Equation of Seawater 2010. Definitions of Practical and Absolute Salinity.
- Randy Holmes-Farley, Refractometers and Salinity Measurement. Applied aquarium reference for measurement scales, temperature and calibration.