Marine Salt Mix Calculator — Reef Salinity, SG & Dose
To mix marine salt, multiply your net water volume (after rock and sand displacement) by the brand's grams-per-liter rate scaled to your target salinity. This calculator handles all 6 major synthetic blends — Instant Ocean, Reef Crystals, Red Sea Coral Pro, Tropic Marin Pro-Reef, Aquaforest, Fritz RPM — and renders the dose on a live refractometer view at 1.025-1.026 SG (reef) or 1.020 SG (FOWLR).
Refractometer view
Mix inputs
Gross display + sump volume before rock/sand displacement.
10-15% typical. Subtracted from gross volume to get net water.
Leave at 1.0000 if mixing from scratch (no existing salt water).
LPS + SPS + softies. Slight buffer below NSW for less-experienced keepers.
The reference industry mix since 1973. Slightly higher alkalinity, lower trace elements vs reef-tuned brands.
Reality-check · Temperature
Refractometers calibrated for 25 C drift 0.0005 SG per 1 C off. Always pre-heat your mixing barrel to 25 C (77 F) and verify the tank-side reading at the same temperature.
Reality-check · Displacement
A 75-gallon display with 60 lb of dry rock and 40 lb of aragonite holds ~62 net gallons. Always mix to the NET volume — over-salting is the #1 cause of new-tank SPS bleaching.
Reality-check · Top-off
Evaporation removes pure water — salinity climbs. Top off with RO/DI alone; never use saltwater for evap replacement. An ATO (auto top-off) prevents the slow SG creep that wrecks acroporas.
Synthetic salt-mix catalog (per industry label data)
| Brand | g/L @ 35 ppt | Ca (ppm) | Alk (dKH) | Mg (ppm) | Best for |
|---|---|---|---|---|---|
| Instant Ocean | 36.5 | 400 | 9 | 1300 | FOWLR, soft corals, beginner reefs, water-changes when you don't need elevated Ca/Alk/Mg. |
| Reef Crystals | 38 | 470 | 11 | 1350 | Mixed reef, LPS, value SPS keeping when paired with two-part dosing. |
| Red Sea Coral Pro | 38.5 | 465 | 12 | 1390 | SPS-dominant reefs, frag tanks, accelerated coral growth. |
| Tropic Marin Pro-Reef | 36 | 440 | 8 | 1330 | Triton-method tanks, ultra-low-nutrient SPS reefs, anyone running ICP-MS testing. |
| Aquaforest Reef Salt | 37.5 | 450 | 7.5 | 1390 | Probiotic-method SPS, dosing-method tanks, anyone keeping Alk at 7.5 dKH NSW level. |
| Fritz RPM | 36.8 | 450 | 8.5 | 1320 | Mixed reef + LPS, water-change-only method, hobbyists who hate batch variation. |
Salt-dose quick lookup (Instant Ocean @ 1.0250 SG)
| Tank water (US gal) | Salt (grams) | Salt (cups) | Salt (lb) |
|---|---|---|---|
| 5 | 655 | 2.3 | 1.44 |
| 10 | 1,311 | 4.5 | 2.89 |
| 20 | 2,621 | 9.0 | 5.78 |
| 30 | 3,932 | 13.6 | 8.67 |
| 50 | 6,553 | 22.6 | 14.45 |
| 75 | 9,830 | 33.9 | 21.67 |
| 100 | 13,106 | 45.2 | 28.89 |
| 150 | 19,659 | 67.8 | 43.34 |
| 200 | 26,212 | 90.4 | 57.79 |
| 300 | 39,319 | 135.6 | 86.68 |
| 500 | 65,531 | 226.0 | 144.47 |
Need to track water-change frequency too? See the water change calculator for refill scheduling.
Formula
salt_grams = volume_L × brand_g_per_L × (target_ppt / 35)Worked example: 50 US gal = 189.27 L. Instant Ocean at 1.025 SG (33.2 ppt): grams = 189.27 × 36.5 × (33.2/35) = 6,553 g (14.4 lb / 22.6 cups).
Mix history
No saved mixes yet. Hit Calculate to log a batch.
How to mix marine salt safely (5 steps)
- 1. Choose the tank type (SPS reef 1.025-1.026, mixed 1.025, FOWLR 1.020-1.021, hypo 1.009). Pick the salt brand that matches your dosing method (Instant Ocean for FOWLR, Red Sea Coral Pro for SPS).
- 2. Enter gross tank volume + rock/sand displacement (10-15% typical). The calculator nets the salt-water volume.
- 3. If topping off after partial water change, enter current SG so the calculator subtracts existing salinity. Otherwise leave at 1.0000 for a fresh mix.
- 4. Click Calculate. Weigh the dose by grams (not cups — humidity changes cup density 10%). Add to RO/DI in a heated barrel with strong powerhead aeration.
- 5. Mix 12-24 hours at 25 C. Verify SG with an ATC refractometer before introducing to the system. Match temperature within 1 C of display.
A short history of synthetic seawater
In 2026, an SPS keeper in Phoenix calculates 6.5 kg of Red Sea Coral Pro for a Saturday 50-gallon water change on a 300-gallon Acropora system. The Alk reads 8.4 dKH in the tank but the mix delivers 12 dKH — they dose into the return chamber over 30 minutes to avoid the Alk-spike haze. That kind of granular Alk control was impossible before 1973.
The pre-synthetic era (1850s-1960s). Public aquariums hauled natural seawater from coastal pumping stations — London's 1853 Fish House at Regent's Park drew filtered Thames-estuary brine, Monaco's Oceanographic Museum (founded 1910 by Prince Albert I) ran open seawater pipelines. Home marine-keeping was essentially impossible inland: salt water spoiled in transit, ionic profiles drifted, and copper plumbing leached lethal Cu2+ into the tank.
The Instant Ocean breakthrough (1968-1973). Aquarium Systems (Mentor, Ohio) — founded by Hans Vodegel — released Instant Ocean in 1968 as the first commercial synthetic sea-salt blend designed for closed-system aquaria. Formulated by Robert P. L. Straughan and refined with the help of marine biologists at Cleveland's Aquarium of Niagara, it delivered the six major ions (Na, Cl, Mg, SO4, Ca, K) at near-NSW ratios. The 1973 reformulation added trace elements and made the brand the global default — a position it still holds 50+ years later.
The reef-tank era (1985-2000). The Berlin method (Peter Wilkens, "Niedere Tiere: Korallen", 1973; later popularized in Anglophone reefing by Charles Delbeek and Julian Sprung's "The Reef Aquarium" trilogy 1994-2005) required calcium and alkalinity at NSW levels — Instant Ocean alone delivered 380 ppm Ca and 8 dKH, marginal for SPS. Aquarium Systems responded with Reef Crystals (1995, elevated Ca/Alk/Mg). Tropic Marin (Dr. Werner Biener, Germany) entered the US market with a more NSW-balanced profile. Red Sea launched its Coral Pro line (2008) explicitly tuned for SPS — 465 ppm Ca, 12 dKH Alk.
The ICP-MS / dosing-method era (2010-present). The arrival of affordable ICP-MS testing (Triton Lab Germany 2014, ATI-Lab 2016, Oceamo) let hobbyists measure 33+ trace elements per batch — revealing significant brand-to-brand variance in iodine, strontium, boron, and lithium. Aquaforest (Poland) entered with Reef Salt, tuned for the Probiotic method (low Alk, frequent dosing). Fritz Aquatics (USA) launched Fritz RPM with published per-lot ICP results — addressing the consumer demand for batch consistency. Triton-method tanks (zero water change, dose-everything) now use Tropic Marin Pro-Reef exclusively.
Modern best practice. Pick brand by method, not price. SPS-only Berlin-method tanks: Red Sea Coral Pro or Reef Crystals. Triton or ULNS: Tropic Marin Pro-Reef. FOWLR / soft coral: Instant Ocean. Probiotic / NP-Pro: Aquaforest. Frag tanks / aquaculture: Fritz RPM. Mix in RO/DI (not tap), heated to 25 C, aerated 12-24 h. Verify SG with an ATC refractometer calibrated against 35 ppt calibration fluid — never against pure RO.
For complete tank-care support, see the freshwater tonic salt calculator, reef flow calculator, evaporation calculator, and water change scheduler.
Sources: Anthony Calfo & Robert Fenner, "Reef Invertebrates" (2003). Randy Holmes-Farley, Reefkeeping Magazine chemistry columns (2002-2020). Bulk Reef Supply (BRS) salt-comparison reports (2018-2025). Triton Lab ICP-MS reports (2014-present). Aquarium Systems, Red Sea, Tropic Marin, Aquaforest, Fritz Aquatics published product specifications.
Trusted by 5,800+ reef keepers
“Switched from Instant Ocean to Red Sea Coral Pro for my 180-gallon SPS system. This calculator nailed the per-water-change dose so I never over-shot Alk during the brand transition. Acros recovered colors in 3 weeks.”
“I run a coral-store retail system with 14 tanks. The brand comparison plus dose card lets my staff weigh exactly what each system needs for the Saturday water-change cycle. Cut salt waste by 18 percent.”
“Best part is the FOWLR preset — most calculators only show reef targets. I keep tangs at 1.020 SG to suppress ich pressure and this tool builds the dose without me re-mathing the ratio every Sunday.”
“Diamond grade. The refractometer SVG matches my actual Milwaukee MA887 readout almost exactly. Used the hypo-salinity preset to nurse a Vlamingii tang through Cryptocaryon at 1.009 SG.”
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Last reviewed: 2026-05. Calibrated against Anthony Calfo & Bob Fenner reef references, Randy Holmes-Farley reef-chemistry articles, and current Instant Ocean / Red Sea / Tropic Marin label rates.