The Importance of Specific Gravity (Salinity Concentration) in Coral Tanks
Among water quality parameters in coral tanks, specific gravity (or salinity concentration) is one of the most critical. Specific gravity is an indicator of the amount of dissolved salts (primarily sodium chloride, along with various ions such as Mg, Ca, K, and SO₄) dissolved in water. It directly affects the osmotic pressure regulation of corals, fish, and other invertebrates.
Relationship Between Specific Gravity and Salinity Concentration
| Specific Gravity (25°C) | Salinity Concentration (PPT/ppt) | Overview |
|---|
| 1.020 | ~28 ppt | Low (therapeutic range for marine fish) |
| 1.023 | ~30 ppt | Lower limit of standard seawater |
| 1.025 | ~33 ppt | Target value for typical seawater and coral keeping |
| 1.026 | ~35 ppt | Average natural seawater value |
| 1.028 | ~37 ppt | High (typically avoided in coral keeping) |
The recommended specific gravity for most coral tanks is 1.025–1.026 (salinity concentration of 33–35 ppt). This range most closely approximates the natural coral reef environment.
Damage to Corals from Specific Gravity Fluctuations
A commonly overlooked aspect of coral damage is "sudden fluctuations in specific gravity." Like water temperature and KH, rapid changes in specific gravity are a stress factor for corals.
Specific Gravity Rise Due to Evaporation
Tank water gradually decreases each day due to evaporation (approximately 0.5–2 liters per day in a 100-liter tank). Only water (H₂O) evaporates—salts do not. Without replacement, specific gravity gradually rises.
In a 100-liter tank evaporating 1 liter daily, specific gravity can rise by 0.002–0.003 over a month. This often causes coral retraction and failure of polyps to open.
Specific Gravity Fluctuation During Water Changes
When replacing water, if the replacement water's specific gravity differs significantly from the current tank water, a sudden change occurs. Always verify replacement water's specific gravity with a hydrometer and adjust it to match the current tank reading before adding it to the tank.
Specific Gravity Measurement Methods and Equipment Selection
The accuracy of specific gravity measurement varies significantly depending on the equipment used.
Swing-Arm (Floating Needle) Hydrometer
The most common and inexpensive (¥1,000–3,000) type of hydrometer. It uses a dial reading system and is simple to use. However, accuracy is limited (±0.001–0.002) and periodic calibration is necessary.
Caution: Air bubbles can cause incorrect readings. Fill the container with water and remove air bubbles before measuring.
Collapsible (Swing-Arm) Hydrometer
Inexpensive but low precision (±0.001–0.002), making it unsuitable for accurate coral keeping. Recommend using it as an entry-level tool while transitioning to higher-precision equipment.
Refractometer (Optical Hydrometer)
Mid-range price (¥3,000–8,000) with high accuracy. Place 1–2 drops of water on the prism, illuminate with light, and read the specific gravity from the position of the boundary line.
Recommended: RHS Type (Marine ATC Refractometer)
Choose one with ATC (Automatic Temperature Compensation) to ensure accurate measurements unaffected by water temperature. Regular calibration (zeroing with RO pure water) is essential.
Digital Specific Gravity Meters / Salinity Meters
The most accurate equipment (±0.0001–0.0002), though higher in price (¥8,000–30,000). Provides precise management. Milwaukee MA887 and Hanna Instruments products are widely used.
Recommended Options
- Beginner to intermediate: ATC optical refractometer (¥3,000–5,000)
- Serious SPS keepers: Digital salinity meter (¥10,000+)
Stabilizing Specific Gravity with ATO (Auto Top-Off)
The most effective method to prevent specific gravity rise from evaporation is an ATO (Auto Top-Off) system.
How ATO Works
A water level sensor detects when the tank (or sump) water level drops, automatically replenishing with RO/DI pure water. This continuously compensates for evaporation, keeping specific gravity stable.
Comparison of Major ATO Systems
| Product | Price Range | Features |
|---|
| Tunze Osmolator | ¥15,000–25,000 | High reliability; most popular |
| AutoAqua Smart ATO | ¥8,000–15,000 | Good value; widely adopted |
| Neptune Systems DOS/ATK | ¥20,000–40,000 | Smart integration with Apex controller |
| DIY (relay + float switch) | ¥3,000–8,000 | DIY option; requires verification |
ATOs are designed exclusively for pure water replenishment, not for artificial seawater (which would cause excessive salinity). This is a fundamental principle.
Preparing Artificial Seawater and Managing Specific Gravity
Here's the procedure for preparing fresh artificial seawater (for water changes or initial tank setup) with proper specific gravity management.
Steps
- Use RO/DI Pure Water: Tap water contains chlorine and TDS; RO/DI water is ideal
- Measure Artificial Seawater Salt: Weigh the manufacturer's specified amount (typically 33–36 g/L)
- Stir and Dissolve: Agitate thoroughly with a circulation pump or air pump (30–60 minutes)
- Adjust Temperature: Match the tank's water temperature (especially important during seasonal changes)
- Measure Specific Gravity: Confirm 1.025–1.026 using a refractometer or digital meter
- Adjust: Add salt if low, or pure water if high; remeasure
- Add to Tank: Add slowly only after the specific gravity difference from existing tank water is within 0.001
The Relationship Between Specific Gravity and Water Temperature
Water's specific gravity changes with temperature (higher temperature = lower specific gravity). This is why the ATC (Automatic Temperature Compensation) function is critical.
Example: Water at 26°C and water at 20°C with identical salinity will show different hydrometer readings. Always measure coral tank water at the same temperature as the tank (or apply temperature correction to readings).
General refractometer temperature correction table (reference):
| Measurement Water Temperature | Correction to Reading |
|---|
| 20°C | −0.0004 (subtract from reading) |
| 25°C | No correction (standard temperature) |
| 30°C | +0.0006 (add to reading) |
With an ATC refractometer, this correction is performed automatically.
Long-Term Management Checklist
Summary of daily, weekly, and monthly checks to maintain stable specific gravity.
Daily
- [ ] Confirm ATO operation (check that supplement tank water level has decreased)
- [ ] Visually inspect tank water level for abnormalities
Weekly
- [ ] Measure specific gravity with refractometer or digital meter (target: 1.025–1.026)
- [ ] Refill the ATO pure water supplement tank
Monthly
- [ ] Calibrate refractometer (verify zero point with RO pure water)
- [ ] Verify ATO system operation and clean sensors
- [ ] Check specific gravity before and after water change
Summary
While often overlooked, specific gravity management in coral tanks is critical to successful keeping. By combining ATO system installation to prevent evaporation-driven specific gravity rise with regular measurement using a high-precision refractometer, you can maintain specific gravity in the stable range of 1.025–1.026. Always verify specific gravity during water changes and avoid sudden changes—this is key to minimizing stress on corals. Stable specific gravity management is the foundation for maintaining corals in beautiful condition long-term.
