Measuring Reef Aquarium Salinity: Density, SG and Calibration
When water evaporates from your reef aquarium, the salts remain and salinity rises. Replace evaporation losses with suitable fresh water, usually RO/DI water, rather than newly mixed saltwater. A water change is different: you remove salt as well as water, so replace that water with appropriately mixed saltwater. Regular measurements help you check that top-ups and water changes are keeping salinity stable.
This guide helps you identify what your instrument measures and investigate conflicting results before changing your aquarium water.
Salinity, density and SG: What does your instrument show?
- Salinity: Describes salt content, often displayed in the hobby as ppt or ‰. Record the unit printed on your scale.
- Density: Mass per volume, for example kg/l. The value depends on temperature.
- Specific gravity (SG): A ratio to the density of reference water, without a unit. The temperature convention used by the scale is part of the result.
Example: Seawater at 35 ‰ has a density of about 1.023 kg/l at 25 °C. A reading of 1.023 on an SG scale means something different. Record a result such as “35 ppt” or “1.023 kg/l at 25 °C”, rather than just a number. This makes comparisons between instruments useful.
Calibrating a refractometer: Follow its instructions
Before buying, check whether the instrument is designed for seawater or a pure salt solution. Their scales are not automatically interchangeable. Do not apply one conversion number from a forum to every model.
Clean the prism and cover before calibrating. Allow the instrument to reach the temperature specified in its instructions and use the required liquid: depending on the model, this may be pure water for setting zero or a suitable salinity standard. Wait the stated time, read the light–dark boundary and adjust as instructed. “ATC” compensates for temperature effects within the instrument’s specified range; it does not replace calibration or a clean prism.
Hydrometers and digital salinity testers
Let a floating hydrometer float freely in its cylinder without touching the wall or bottom. Remove attached air bubbles and check the water temperature, because the scale uses a specified temperature. Read at the point required by your model’s instructions: the water surface or the meniscus. Reading at a different point can change the result.
Calibrate a digital salinity tester with the solution specified for its measurement method. Rinse and care for the sensor according to its instructions, and wait for the reading to stabilise. A reference intended for a refractometer is not automatically suitable for a conductivity meter just because both say “35 ppt”.
Two instruments disagree: Our checking sequence
- Record the model, unit, reading and water temperature. Do not compare kg/l directly with SG.
- Check cleaning, calibration and the reference against each instrument's instructions.
- Measure a fresh sample again and record both results.
- If the difference persists, arrange an independent cross-check. Do not change the tank based only on an unexplained single reading.
Frequently asked questions
Is 1.023 too low for a reef aquarium?
It cannot be assessed without the unit and temperature. First establish whether the instrument displays density or SG.
Can I use any 35-ppt reference?
Check that the manufacturer and reference specify compatible measurement and calibration conditions. The number alone is insufficient.
Which instrument should I choose?
Choose a seawater instrument with instructions, a scale and a reference you can understand. A simple, repeatable routine matters more than another decimal place alone.
Browse our water tests and measuring equipment. For ongoing care, read keeping alkalinity stable and ICP analysis or home testing. Ask us about your specific instrument through our contact page.
