Saltwater Reef Tank Salt Mix Calculator
Calculate exactly how much salt mix to add to your aquarium for a target salinity or specific gravity.
For marine, reef, and FOWLR tanks.
Saltwater aquariums require precise salinity to keep fish and corals healthy. Natural seawater has a salinity of about 35 ppt (parts per thousand), which equals a specific gravity of 1.025–1.026 at 77°F (25°C).
Key salinity targets:
| Tank Type | Salinity (ppt) | Specific Gravity |
|---|---|---|
| Fish Only (FOWLR) | 30–35 ppt | 1.022–1.025 |
| Reef / Coral Tank | 34–36 ppt | 1.025–1.026 |
| Natural Sea Water (NSW) | 35 ppt | 1.025–1.026 |
| Hyposaline (ich treatment) | 14–16 ppt | 1.010–1.012 |
The formula:
Salt needed (grams) = Volume (liters) × Target salinity (ppt)
Or in imperial:
Salt needed (cups) ≈ Volume (gallons) × 0.5 cups per gallon per 35 ppt
This is because most commercial salt mixes dissolve to approximately 35 ppt at 0.5 cups per US gallon of RODI water. Exact amounts vary slightly by brand — always verify with a refractometer or hydrometer.
Why RODI water matters:
RODI stands for Reverse Osmosis Deionized water. Using tap water introduces phosphates, nitrates, and chlorine which harm corals and beneficial bacteria. Always mix salt with RODI water for best results.
How to mix properly:
- Fill a clean bucket with RODI water at room temperature
- Add salt gradually while stirring or using a powerhead
- Let the salt dissolve fully for at least 30–60 minutes
- Measure salinity with a refractometer (more accurate than a swing-arm hydrometer)
- Adjust with more salt or more RODI water to hit your target
- Let the water aerate overnight before adding it to the tank
Measuring tools:
- Refractometer: Most accurate — measures light refraction through the water
- Digital salinity meter: Very accurate, but calibration is important
- Swing-arm hydrometer: Least accurate, prone to bubbles and drift — not recommended for reef tanks
Common salt mix amounts (approximate):
| Volume | Salt for 1.025 SG |
|---|---|
| 1 gallon (3.8 L) | ~1/2 cup (~130 g) |
| 5 gallons (19 L) | ~2.5 cups (~650 g) |
| 10 gallons (38 L) | ~5 cups (~1,300 g) |
| 25 gallons (95 L) | ~12.5 cups (~3,250 g) |
| 50 gallons (190 L) | ~25 cups (~6,500 g) |