Aquarium Salinity Converter
Convert between aquarium salinity units including specific gravity, PPT (parts per thousand), PSU, and conductivity.
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Salinity measurement is essential for maintaining a healthy saltwater or brackish aquarium. There are several different units used to express salt concentration in water, and converting between them is a common task for aquarists.
Common salinity units:
- Specific Gravity (SG): The density of saltwater compared to pure water. Pure water has an SG of 1.000. Typical reef aquarium SG is 1.023-1.026.
- Parts per Thousand (PPT or ppt): The grams of dissolved salt per kilogram of water. Also written as “per mille” (using the symbol similar to percent but with two zeros).
- Practical Salinity Units (PSU): Numerically almost identical to PPT for seawater. Based on conductivity measurements.
- Conductivity (mS/cm): Millisiemens per centimeter. Measures the electrical conductivity of the water, which correlates with salt content.
Conversion formulas (approximate, at 25 degrees Celsius): The relationship between specific gravity and salinity is not perfectly linear, but a widely used approximation is:
- PPT = (SG - 1) x 1328.57
- SG = 1 + (PPT / 1328.57)
- PSU is approximately equal to PPT for seawater in the normal range
- Conductivity (mS/cm) = PPT x 1.5 (rough approximation at 25 degrees Celsius)
Target salinity ranges for aquariums:
| Environment | SG | PPT/PSU | Conductivity (mS/cm) |
|---|---|---|---|
| Freshwater | 1.000 | 0-0.5 | 0-0.8 |
| Brackish (low) | 1.002-1.008 | 3-10 | 4-15 |
| Brackish (high) | 1.008-1.018 | 10-24 | 15-36 |
| Reef aquarium | 1.023-1.026 | 33-35 | 50-53 |
| Natural ocean | 1.025 | 35 | 53 |
| Hypersaline | 1.030+ | 40+ | 60+ |
Measuring salinity accurately:
- Refractometers measure specific gravity directly and are the most popular tool among reef hobbyists. They should be calibrated with a reference solution (not just freshwater) for best accuracy.
- Hydrometers (floating or swing-arm) also measure specific gravity but are less accurate than refractometers. Temperature affects readings significantly.
- Digital salinity meters measure conductivity and convert to PSU or PPT internally. They are the most accurate when properly calibrated.
Temperature effects: Specific gravity changes with temperature because water density changes. Most conversion tables and refractometers are calibrated for 25 degrees Celsius (77 degrees Fahrenheit). If your water temperature is significantly different, readings may need correction. A 5-degree Celsius difference can shift SG readings by approximately 0.001.
Why stable salinity matters: Marine fish and invertebrates are sensitive to salinity changes. Rapid changes can cause osmotic stress, leading to illness or death. When topping off evaporated water, always use freshwater (not saltwater) because salt does not evaporate. When performing water changes, match the new saltwater to the tank’s salinity before adding it. Aim to keep salinity stable within 0.001 SG (about 1 PPT) from day to day.