Mixing Synthetic Saltwater for Marine Aquariums

How to prepare artificial seawater from synthetic salt: using purified water, target salinity, measuring with a calibrated refractometer, mixing and ageing, and temperature-matching.

Overview

Marine aquariums are filled and maintained with artificial seawater made by dissolving a synthetic salt mix in purified water. A good mix reproduces the salinity and major ion balance of natural seawater closely enough for fish, corals and invertebrates. The key steps are using the right water, hitting the correct salinity, mixing thoroughly, and letting the new water stabilise before it is used.

Start with purified water

Synthetic salt is dissolved in purified water, typically reverse-osmosis/deionised (RO/DI) water, rather than tap water. Tap water can carry chlorine, chloramine, phosphate, nitrate, metals and other contaminants that feed algae or harm invertebrates, so starting from purified water keeps the made-up seawater clean and consistent batch to batch.

Target salinity

Natural seawater has a salinity of about 35 parts per thousand, corresponding to a specific gravity of roughly 1.026 at 25 °C. Reef tanks are commonly kept at natural-seawater salinity (about 35 ppt, around 1.025 to 1.026 specific gravity), and reef sources often cite 35 ppt / 1.0264 as the reference point; fish-only systems are sometimes run slightly lower. The important thing is to mix to a consistent target and keep it stable.

Measuring salinity

Salinity is usually measured with a refractometer, which must be calibrated to be accurate. Reef chemists note that calibrating only with pure (zero) freshwater can leave a refractometer reading several parts per thousand off at seawater strength, so calibration against a known seawater-strength reference solution near 35 ppt is recommended, with the instrument at room temperature. Conductivity meters and quality hydrometers are also used.

Mixing and ageing

• Add salt to the water (not water to salt) with strong circulation from a powerhead and a heater running.
• Mix to the target salinity, adjusting with more salt or more RO/DI water as needed.
• Let the batch circulate and age, commonly about 24 hours, until it is clear, fully dissolved and equilibrated.
• Match the new water's temperature and salinity to the tank before using it for a water change.

Using the water

Once the batch is clear, at the target salinity and matched to tank temperature, it is ready for a water change or to fill a new system. Keeping a consistent salt brand and mixing routine helps every batch match the tank, so water changes do not cause sudden shifts in salinity or chemistry that stress sensitive corals and invertebrates.