Can I add salt directly to the display tank?

No. Always pre-dissolve in 1 gallon of fresh saltwater or RO/DI, then drip slowly into a high-flow zone (skimmer intake, sump return). Dry salt grains sitting on coral cause tissue damage.

Should I use reef salt or table salt?

Reef salt only. Table salt is pure NaCl and lacks the calcium, magnesium, and trace elements your reef needs. Same for kosher / sea salt / pickling salt — none have the right composition.

What if I overshoot the target?

Reverse with a small RO/DI top-off over the next 24 hours. Same 1 ppt/day swing limit applies in the reverse direction. Lower salinity is generally better tolerated than higher.

Calculator

Direct Salt Addition Calculator

Name: Direct Salt Addition Calculator
Author: reefcalcs

Sometimes salinity drifts low — over-aggressive RO/DI top-off, a leaky ATO that ran on freshwater, a recent water change with under-mixed new salt. The fix is direct salt addition: dissolve dry reef salt in fresh saltwater (or RO/DI), then drip-introduce. Easy math, but easy to overshoot.

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.

Direct salt addition

Volume (US gal)

Current salinity (ppt)

Target salinity (ppt)

Salt to add

568g

In ounces

20.04oz

In cups (approx)

2.37cups

~240 g per cup of dry reef salt.

⚠ Swing of 2.0 ppt exceeds safe daily limit (1 ppt). Split the addition across 2 days.

Salinity (ppt) ≈ grams of dissolved salt per litre. Dissolve salt in 1 L of fresh saltwater (or RO/DI) before adding to the tank, drip slowly into a high-flow zone, and cap single-day swings at 1 ppt.

How this is calculated

Salinity in ppt ≈ grams of dissolved salt per litre.

delta_ppt = target_ppt − current_ppt
total_grams = delta_ppt × tank_L

Tank litres = tank_gal × 3.78541

Conversions:
  ounces = grams × 0.035274
  cups   = grams ÷ 240   (Red Sea / Tropic Marin dry salt density)

Safety cap: maximum 1 ppt salinity swing per day.

The 1 ppt/day cap is conservative — fish handle larger swings short-term, but osmotic stress builds slowly. Reef inverts (snails, shrimp, corals) are more sensitive. If your salinity is > 2 ppt off, split the correction over 2–3 days and dose at night when the tank is calmest.

Always pre-dissolve salt in fresh saltwater or RO/DI before adding to the tank. Dropping dry salt directly into the display creates local salinity spikes that burn coral tissue.

FAQ

Can I add salt directly to the display tank?: No. Always pre-dissolve in 1 gallon of fresh saltwater or RO/DI, then drip slowly into a high-flow zone (skimmer intake, sump return). Dry salt grains sitting on coral cause tissue damage.
Should I use reef salt or table salt?: Reef salt only. Table salt is pure NaCl and lacks the calcium, magnesium, and trace elements your reef needs. Same for kosher / sea salt / pickling salt — none have the right composition.
What if I overshoot the target?: Reverse with a small RO/DI top-off over the next 24 hours. Same 1 ppt/day swing limit applies in the reverse direction. Lower salinity is generally better tolerated than higher.

Where to buy

Use the same salt mix you usually use for water changes so the chemistry stays consistent.

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

Best per-gallon value for SPS tanks.

View on Amazon →

Tropic Marin Pro Reef Salt

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

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.

<iframe
  src="https://reefcalcs.com/embed/direct-salt/"
  width="100%"
  height="720"
  style="border: 1px solid #1C2D34; border-radius: 8px; max-width: 720px;"
  loading="lazy"
  title="ReefCalcs Direct Salt Addition Calculator"
></iframe>

Sources & references

01
Hamza's Reef — direct salt addition calculator (methodology reference)
https://www.hamzasreef.com/Contents/Calculators/DirectSaltCalculator.php
02
Randy Holmes-Farley — Specific gravity and saltwater chemistry
https://reefs.com/magazine/the-relationship-between-alkalinity-and-ph/

Last reviewed 2026-05-25