How much salt do I need for a 50-gallon water change?

About 7.2 kg / 16 lb of Red Sea Salt or 7.0 kg of Reef Crystals to mix 50 US gallons at 1.025 SG. Roughly 25 US cups by volume — but weigh it for accuracy.

Should I use 1.025 or 1.026 SG for a reef tank?

1.025–1.026 SG (34–35 ppt) matches the open Pacific reefs most aquacultured corals come from. Below 1.023 stresses LPS/SPS; above 1.027 is fine for fish-only but slows coral growth slightly. Pick a number and hold it within ±0.001.

Why does Red Sea Coral Pro need more salt per gallon?

Coral Pro is formulated with elevated alkalinity (12.2 dKH), calcium (465 ppm), and magnesium (1390 ppm). More dissolved solids means more grams of dry salt per litre — roughly 41 g/L vs. 38 g/L for standard reef salts.

Can I use kitchen tap water?

No. Tap water carries chlorine, chloramine, phosphate, silicate, copper, and dissolved organics. Even one mix with municipal tap can seed a months-long algae bloom. RO/DI only — every reefer learns this twice.

How long should I let the salt mix before using it?

Minimum 24 hours with a powerhead running, heated to display temperature. Some salts (Red Sea Coral Pro, Tropic Marin syn-biotic) precipitate calcium carbonate if you skip the mix-time — you'll see a snow effect that costs you 1 dKH per batch.

Refractometer or hydrometer?

Refractometer, calibrated monthly with 35-ppt calibration fluid. Swing-arm hydrometers are accurate to ±0.002 SG out of the box and drift fast with salt creep. The math here assumes refractometer-level accuracy.

Calculator

Reef Salt Mix Calculator

Name: Reef Salt Mix Calculator
Author: reefcalcs

Salt-mix math is straightforward in principle — grams of salt per litre of RO — but every brand publishes its own dose because grain size, density, and supplemental package vary. A scale beats a measuring cup every time; cup measures drift ±10% with brand and how compactly the salt was packed at the factory.

reefcalcs editorialUpdated May 25, 2026Reviewed for accuracy May 15, 20266 sources cited

JavaScript is disabled in your browser, so the interactive calculator below won't run. The math, formula, and worked example are all written out further down this page under “How this is calculated.” Enable JS (or open this page in another browser) to use the live calculator.

Whether you compute by hand or in the calculator, the same caveat applies: verify against your test kit and the linked primary source before dosing. reefcalcs is reference information, not veterinary or engineering advice.

Salt mix calculator

Water volume (US gal)

Salt brand

Target salinity (SG)

Mixed reef: 1.025–1.026 SG (33–35 ppt). FOWLR: 1.022–1.024.

Dry salt — grams

6812g

Dry salt — kg

6.81kg

Dry salt — lb

15.02lb

Approx. cups

23.7US cups

Volumetric estimate — weigh whenever possible.

Target salinity

33.1ppt

Brand

Red Sea Salt

Red Sea Salt — Red Sea Aquatics datasheet

Resulting batch parameters (Red Sea Salt)

Alkalinity

8.2 dKH

Calcium

430 ppm

Magnesium

1340 ppm

Published manufacturer values at this brand's reference mix (~35 ppt). Test the finished batch — real values drift ±5%.

Salt for your next mix:

Red Sea Coral Pro Salt — 22 kg bucket (175 gal mix)

~$90 · View on Amazon →

Instant Ocean Reef Crystals — 200 gal box

~$55 · View on Amazon →

Tropic Marin Pro Reef Salt

~$110 · View on Amazon →

Affiliate links. As an Amazon Associate, reefcalcs earns from qualifying purchases.

How this is calculated

grams_per_L      = brand_g_per_L_at_35ppt × (target_ppt / 35)
total_grams_salt = grams_per_L × water_litres
cups_salt        = brand_cups_per_gal_at_1.025 × gallons × (target_ppt / 35)
target_ppt       = (target_SG - 1) × 1326    // hobbyist approximation

Per-brand dose constants

Each salt has a published "reference dose" on the bucket — usually grams per litre or cups per gallon — to reach roughly 35 ppt / 1.025 SG. The calculator scales linearly from that reference to your target salinity. Real chemistry is slightly non-linear at the extremes (below 1.020 or above 1.030), but ±2% accuracy is plenty for a reef batch you're going to verify with a refractometer anyway.

Instant Ocean / Reef Crystals — Spectrum Brands. IO targets 33–35 ppt; Reef Crystals is the same recipe with elevated calcium and trace elements for reef tanks. Instant Ocean product page.
Red Sea Salt and Red Sea Coral Pro — Red Sea Aquatics. Coral Pro is the "elevated" version: 12.2 dKH, 465 ppm Ca, 1390 ppm Mg at the reference mix. Red Sea Salt range.
Tropic Marin Classic / Pro-Reef — German lab salt, lower published alk (7.5–8 dKH) than Red Sea but with a very clean trace-metal profile. Tropic Marin sea salts.
Fritz RPM Reef Pro Mix — US-made, mixed-reef parameters (8.5 dKH, 450 ppm Ca, 1350 ppm Mg). Fritz RPM product page.

Why weigh instead of measure cups

Cup measures assume a fixed bulk density. Real salt buckets settle in shipping: the top of a fresh bucket is about 10–15% lighter per cup than the bottom of an older bucket. The same "½ cup" can deliver 1.024 or 1.027 depending on how the bag was packed, shaken, or stored. A scale removes that error. For batches under 10 gallons it's the difference between coral pop and a slow-burn drift in salinity over a year of top-offs.

FAQ

How much salt do I need for a 50-gallon water change?: About 7.2 kg / 16 lb of Red Sea Salt or 7.0 kg of Reef Crystals to mix 50 US gallons at 1.025 SG. Roughly 25 US cups by volume — but weigh it for accuracy.
Should I use 1.025 or 1.026 SG for a reef tank?: 1.025–1.026 SG (34–35 ppt) matches the open Pacific reefs most aquacultured corals come from. Below 1.023 stresses LPS/SPS; above 1.027 is fine for fish-only but slows coral growth slightly. Pick a number and hold it within ±0.001.
Why does Red Sea Coral Pro need more salt per gallon?: Coral Pro is formulated with elevated alkalinity (12.2 dKH), calcium (465 ppm), and magnesium (1390 ppm). More dissolved solids means more grams of dry salt per litre — roughly 41 g/L vs. 38 g/L for standard reef salts.
Can I use kitchen tap water?: No. Tap water carries chlorine, chloramine, phosphate, silicate, copper, and dissolved organics. Even one mix with municipal tap can seed a months-long algae bloom. RO/DI only — every reefer learns this twice.
How long should I let the salt mix before using it?: Minimum 24 hours with a powerhead running, heated to display temperature. Some salts (Red Sea Coral Pro, Tropic Marin syn-biotic) precipitate calcium carbonate if you skip the mix-time — you'll see a snow effect that costs you 1 dKH per batch.
Refractometer or hydrometer?: Refractometer, calibrated monthly with 35-ppt calibration fluid. Swing-arm hydrometers are accurate to ±0.002 SG out of the box and drift fast with salt creep. The math here assumes refractometer-level accuracy.

Where to buy

Coral Pro for high-alk SPS tanks, Pro Reef for low-alk dosing systems, Reef Crystals for budget mixed reefs. Don't trust hydrometers — use a calibrated refractometer.

Red Sea Coral Pro Salt — 22 kg bucket (175 gal mix)

Best per-gallon value for SPS tanks.

View on Amazon →

Red Sea Coral Pro Salt — 7 kg bucket (55 gal mix)

High-alk, high-Ca reef salt. Spec sheet at 11.5–12.5 dKH.

View on Amazon →

Tropic Marin Pro Reef Salt

7–8 dKH spec. The go-to for calcium-reactor and Triton tanks.

View on Amazon →

Instant Ocean Reef Crystals — 200 gal box

Cheapest reef salt with decent reef chemistry. Used to start.

View on Amazon →

Refractometer for saltwater + calibration fluid

Calibrate with 35 ppt fluid monthly. Replaces hydrometers.

View on Amazon →

Affiliate links. As an Amazon Associate, reefcalcs earns from qualifying purchases.

Share / embed this calculator

Paste this snippet into any blog or forum post that allows raw HTML to embed the calculator.

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  src="https://reefcalcs.com/embed/salt-mix/"
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Sources & references

01
Instant Ocean / Reef Crystals — mixing instructions
https://www.instantocean.com/products/instant-ocean.aspx
02
Red Sea Salt — product page and parameters
https://www.redseafish.com/salt/red-sea-salt/
03
Red Sea Coral Pro — elevated reef-salt datasheet
https://www.redseafish.com/salt/coral-pro-salt/
04
Tropic Marin sea salts — Classic and Pro-Reef
https://www.tropic-marin.com/meerwasser-produkte?lang=en
05
Fritz RPM Reef Pro Mix — manufacturer page
https://fritzaquatics.com/products/fritz-rpm-reef-pro-mix
06
Randy Holmes-Farley — Specific Gravity in reef aquaria
https://reefs.com/magazine/chemistry-and-the-aquarium-specific-gravity-oh-how-complicated/

Last reviewed 2026-05-25