CALCULATOR · SALT
Reef salt mix calculator
The short answer
To mix ~35 ppt saltwater, use about 0.5 cup (≈136 g) of salt per gallon. A 25-gallon water change needs roughly 3,400 g of mix — about $10–15 depending on brand. Always add salt to RO/DI (never the reverse) and mix to a measured salinity, not a fixed scoop count.
The ReefRig Salt Method
Start at ~0.5 cup (≈136 g) of mix per gallon for ~35 ppt, add to RO/DI (never the reverse), and always mix to a measured salinity — not a fixed scoop count.
A 25-gallon water change takes ~3,400 g (12.5 cups) of salt — about $10 with Instant Ocean, $14 with Red Sea Coral Pro.
How much salt by batch size
Starting rate ≈ 0.5 cup (136 g) per gallon to hit ~35 ppt. Exact rate varies a little by brand, so always confirm with a refractometer and adjust — this table gets you close.
| New saltwater | Salt (grams) | Salt (cups) |
|---|---|---|
| 1 gal | 136 | 0.5 |
| 5 gal | 680 | 2.5 |
| 10 gal | 1,360 | 5 |
| 15 gal | 2,040 | 7.5 |
| 20 gal | 2,720 | 10 |
| 25 gal | 3,400 | 12.5 |
| 50 gal | 6,800 | 25 |
Cost by salt brand
Five mainstream reef salts, with real 2026 per-gallon cost and the alkalinity each mixes to. High-alk salts (Coral Pro, Reef Crystals) suit heavily-dosed SPS; low-alk salts (Tropic Marin, Fritz) suit low-nutrient systems.
| Salt | Mixed alk | $/gal | 25 gal | 50 gal |
|---|---|---|---|---|
| Instant Ocean Reef Crystals | 12 dKH | $0.40 | $10 | $20 |
| Fritz RPM (Reef Pro Mix) | 8.5 dKH | $0.42 | $10.5 | $21 |
| Instant Ocean Sea Salt | 8 dKH | $0.45 | $11.3 | $22.5 |
| Red Sea Coral Pro | 12 dKH | $0.57 | $14.2 | $28.5 |
| Tropic Marin Pro Reef | 7 dKH | $0.60 | $15 | $30 |
Per-gallon cost = mainstream bucket/box price ÷ gallons it makes (data/seed, verified 2026-07-03). Amounts assume the ~136 g/gal general rate; mix to a measured salinity.
LIVE CALCULATOR
Mix a batch
Enter the gallons of new saltwater you’re mixing and pick a salt. Amounts use the ~136 g/gal starting rate; cost uses each brand’s real 2026 per-gallon price.
Starting rate ~136 g (0.5 cup) per gallon. Add salt to RO/DI, run a pump and heater, and confirm ~35 ppt with a refractometer before use — brands vary slightly, so treat this as a starting point.