Saltwater Aquarium Salt Mix Calculator
Calculate the amount of marine salt mix needed for your aquarium based on volume and target salinity.
Saltwater aquariums require precise salinity levels to keep marine fish, corals, and invertebrates healthy. The salinity is controlled by dissolving commercial marine salt mix into purified water (RO/DI — reverse osmosis/deionization). The amount of salt needed depends on the water volume, the target salinity, and the brand of salt mix (each brand has slightly different density).
Salinity Measurement
Salinity is measured in three interchangeable ways:
| Measurement | Natural Seawater | Reef Tank Target | FOWLR Target |
|---|---|---|---|
| Specific Gravity (SG) | 1.025 | 1.024–1.026 | 1.020–1.024 |
| PPT (parts per thousand) | 35 ppt | 33–36 ppt | 28–33 ppt |
| PSU (practical salinity units) | 35 PSU | 33–36 PSU | 28–33 PSU |
FOWLR = Fish Only With Live Rock (fish are more tolerant of salinity variation than corals).
Salt Mix Formula
Salt (cups) = Water Volume (gallons) × Salt Rate (cups/gallon)
| Target Salinity (SG) | Salt Rate (cups/gallon) | Salt Rate (g/liter) | Common Use |
|---|---|---|---|
| 1.020 | 0.42 | 28 | Quarantine, hyposalinity treatment |
| 1.022 | 0.47 | 31 | FOWLR (low end) |
| 1.024 | 0.52 | 34 | FOWLR / reef (versatile) |
| 1.025 | 0.54 | 35 | Natural seawater, reef standard |
| 1.026 | 0.56 | 37 | High-end reef with SPS corals |
These rates are approximate. Different salt mix brands vary by 5–10%. Always measure with a refractometer after mixing.
Mixing Process
- Fill container with RO/DI water at target temperature (75–78°F / 24–26°C)
- Add salt mix gradually while stirring with a powerhead or pump
- Mix for 15–30 minutes until fully dissolved and clear
- Measure salinity with a calibrated refractometer
- Adjust: add more salt to raise SG, or more RO/DI water to lower it
- Aerate for 8–24 hours before adding to the tank (allows pH to stabilize and dissolved gases to equalize)
Worked Example — 75-Gallon Reef Tank, SG 1.025
Water volume: 75 gallons (284 liters). Salt rate: 0.54 cups/gallon. Salt needed: 75 × 0.54 = 40.5 cups (approximately 10 lbs / 4.5 kg).
For a 10% water change on this tank: 7.5 gallons × 0.54 = 4.05 cups of salt mix.
Water Change Salt Calculations
Regular water changes are the foundation of reef tank maintenance. Typical schedule:
| Tank Type | Change Frequency | Volume Changed | Why |
|---|---|---|---|
| Reef (SPS/LPS corals) | Weekly | 10–15% | Replenish trace elements, remove waste |
| Reef (soft corals) | Bi-weekly | 10–20% | Less demanding corals |
| FOWLR | Bi-weekly to monthly | 15–25% | Fish waste management |
| Quarantine | After each treatment | 50–100% | Remove medications |
Temperature Affects Salinity Reading
Specific gravity readings change with temperature. A refractometer calibrated at 77°F (25°C) will read incorrectly if the water is significantly warmer or cooler. Always measure at the calibration temperature or use a temperature-compensating refractometer (most modern ones do this automatically).
RO/DI Water is Essential
Never mix salt with tap water. Tap water contains chlorine, chloramine, phosphates, silicates, and heavy metals that promote algae growth and harm corals. RO/DI systems remove 95–99% of dissolved solids. If you cannot get an RO/DI unit, purchase RO water from your local fish store.