Mixing Saltwater: Salinity, Specific Gravity, and Refractometers
Getting salinity right is the foundation of every marine tank. This guide explains what salinity and specific gravity actually mean, why a refractometer beats a hydrometer, the target range for reef and fish-only tanks, and a clean step-by-step method for mixing consistent saltwater every time.
Quick answer
Mix marine salt into purified (RODI) water, circulate and heat it, then measure salinity with a calibrated refractometer. Aim for roughly 1.024–1.026 specific gravity (about 33–35 parts per thousand) for reef tanks, slightly lower is tolerable for fish-only setups. Always let the mix dissolve fully and reach tank temperature before you test or use it, and match the salinity of any new water to your tank to avoid shocking livestock.
Getting salinity right is the foundation of every marine tank.
Salinity vs specific gravity
Salinity is the actual amount of salt dissolved in water, measured in parts per thousand (ppt). Specific gravity is how dense the water is compared with pure water, which changes with both salt content and temperature. Hobby tools often show specific gravity, but because it drifts with temperature, salinity in ppt (or the temperature-compensated readings on a good refractometer) is the more reliable figure to standardise on.
Why a refractometer beats a hydrometer
Cheap plastic swing-arm hydrometers are notoriously inaccurate, drift with bubbles and residue, and can be off by a wide margin.
A refractometer gives a far more reliable salinity reading than a swing-arm hydrometer.
A refractometer reads salinity from how the water bends light and is far more consistent. Buy one that is temperature-compensated (ATC), and — this is the step most people skip — calibrate it regularly with RODI water or a proper calibration fluid. An uncalibrated refractometer is just a confident hydrometer.
Mixing saltwater step by step
Consistency comes from a repeatable method.
Always add salt to water, circulate, heat, and let it mix fully before testing.
- Start with purified RODI water, not tap water, which adds phosphate and metals.
- Add salt to the water (not water to salt), a bit at a time.
- Run a powerhead and a heater to circulate and warm the mix to tank temperature.
- Let it mix for a few hours, ideally overnight, until fully dissolved and clear.
- Test with your calibrated refractometer and adjust: add salt to raise salinity, add RODI water to lower it.
- Only use the water once it matches your tank's temperature and salinity.
Keeping salinity stable day to day
Evaporation removes water but leaves salt behind, so as the tank evaporates, salinity creeps up. Top up evaporation with plain RODI water, never saltwater. Reserve saltwater only for water changes, where you are replacing water that still contains salt. Mixing these two up is the most common salinity mistake beginners make.
Quick FAQs
Can I use tap water to mix saltwater? It is not recommended. Tap water carries phosphate, silicate, chlorine and metals that fuel algae and stress livestock. Purified RODI water avoids these problems entirely.
How often should I calibrate my refractometer? Regularly — at least monthly, and any time it has been knocked or stored a while. Calibration is quick and it is the difference between a reliable reading and a confident guess.
Why is my salinity slowly rising? Evaporation. Water leaves as vapour but salt stays, concentrating the remaining water. Top up the lost volume with fresh RODI water, not saltwater.
Does temperature affect my salinity reading? Yes, strongly, for specific gravity. Use a temperature-compensated refractometer and let the water reach a stable temperature before reading, or your numbers will mislead you.