How to Make Artificial Seawater and Specific Gravity / Salinity | Basics of Measuring and Adjusting Salinity Concentration

What is Artificial Seawater

In saltwater tanks, rather than using natural seawater directly, the standard practice is to use "artificial seawater" made by dissolving artificial seawater salts (a blend of salts and minerals in powder or granule form) into purified water such as RO/DI water. Natural seawater varies in composition depending on collection location and season, and carries risks of pathogen and contaminant introduction. Artificial seawater, which maintains consistent quality, is therefore widely used.

Basic Steps for Making Artificial Seawater

The basic procedure for making artificial seawater is as follows:

1. Prepare purified source water such as RO/DI water with contaminants removed
2. Measure the specified amount of artificial seawater salt and dissolve it in the source water
3. Stir thoroughly using a circulation pump or similar, ensuring complete dissolution
4. Adjust the water temperature to match that of the tank
5. Check salinity concentration using a hydrometer or refractometer to confirm it falls within the target range

Mixing ratios vary by product, so the first step is to follow the specified amount indicated on your artificial seawater salt package. If any powder remains undissolved, the specific gravity reading will be unstable, so thorough stirring before measurement is essential.

What are Specific Gravity and Salinity Concentration

"Specific gravity" is a value comparing water density to a reference standard (fresh water = 1.000), and seawater has a higher specific gravity than fresh water due to dissolved salts and minerals. Since specific gravity is affected by both salinity concentration and water temperature, measurement evaluation must account for temperature conditions.

The reference specific gravity for coral tanks is approximately 1.023–1.026 (at 25°C), similar to natural coral reef seawater. Soft coral-dominant tanks may be managed at slightly lower ranges, while tanks featuring SPS corals and other species that prefer natural-like conditions may be maintained at slightly higher ranges. In either case, avoiding sudden fluctuations and maintaining stability within a set range takes priority.

Measurement Methods with Hydrometers and Refractometers

There are two main tools for measuring salinity concentration:

• Hydrometer (floating type, swing-arm type, etc.): A simple tool floated in water to read the scale. Convenient to use, but prone to error from air bubbles and insufficient temperature compensation
• Refractometer: A tool that calculates salinity concentration from the refractive index of light. Uses a small water sample and generally provides more stable readings than a hydrometer

Whichever tool you use, regularly calibrating it with a reference solution (a calibration fluid of known concentration) will maintain measurement accuracy. Using the same tool consistently also makes it easier to track changes over time on a relative basis.

Tips for Dissolution and Stirring

Artificial seawater salt often remains incompletely dissolved immediately after being added to source water. Measuring specific gravity with insufficient stirring can yield readings that are lower or unstable compared to the actual value. Using aeration or a circulation pump to stir thoroughly, and confirming the water is clear and no powder remains at the bottom, ensures accurate measurement.

Even when dealing with large volumes or rushing to add water to the tank, do not skip the stirring phase—it is fundamental to stable specific gravity management. A good habit is to recheck the specific gravity after allowing some time from the start of stirring, to verify whether the reading has stabilized. This helps prevent problems after water introduction.

Salinity Concentration Adjustment Methods

If the measured specific gravity falls outside the target range, the basic adjustment methods are:

• If specific gravity is too high: Dilute by adding saltless water such as RO/DI water
• If specific gravity is too low: Add more artificial seawater salt, or prepare separate artificial seawater with higher specific gravity and add it

Any adjustment should be made gradually to avoid stressing corals or fish. Adjustments should be made over time in small increments, including gradual changes to temperature and water chemistry across the entire tank. In particular, if newly made artificial seawater differs significantly in specific gravity from the tank's seawater before a water change, fluctuations will recur with each change. Make it a habit to check the specific gravity of new seawater before performing any water change.

Water Temperature Changes and Salinity Concentration Relationship

Since specific gravity changes with water temperature, when evaluating a measured value, confirm "what temperature the tool is calibrated for." Most hydrometers and refractometers are calibrated at 25°C, so measurements taken at significantly different temperatures will show values that differ from actual salinity concentration. When possible, match the water temperature to conditions similar to the tank before measuring, for more accurate readings.

Water temperature management itself is a separate topic, but since temperature and specific gravity are related factors, recording both during daily water quality checks makes it easier to track overall tank condition. For guidance on water temperature management using heaters and chillers, see Saltwater Tank Temperature Management | Ideal Water Temperature, Heaters, Chillers, and Summer High-Temperature Measures.

Summary

While making artificial seawater itself is straightforward, managing specific gravity and salinity concentration is a fundamental element supporting water quality in coral tanks. By performing regular measurements and calibration with a hydrometer or refractometer, and making adjustments gradually while avoiding sudden fluctuations, you can maintain a stable environment for your corals. For the bigger picture of water quality management, refer to Saltwater Tank Water Chemistry | Basics of KH, Calcium, and Magnesium and Setting Up a Reef Tank | The Journey from Cycling to Initial Maintenance.