reefcalcs

MagnesiumMg

Magnesium is the chemistry parameter most reefers under-test and over-correct. Mg sits at about three times the concentration of calcium by weight — and its job is to inhibit spontaneous CaCO₃ precipitation. Without enough Mg, you cannot maintain reef-level Ca and Alk simultaneously. Test it monthly, dose when needed, and stop poking it.

Target ranges

Beginner-safe1,200–1,400 ppm
Optimal (consensus)1,250–1,400 ppm
Triton method target1,300–1,380 mg/L
Natural seawater~1,284 ppm at 35 ppt (≈ 53 mmol/kg)

Why it matters

Mg²⁺ ions occupy crystal-growth sites on nascent CaCO₃ particles, "poisoning" the surface and preventing further deposition. This is what lets ocean water hold ~412 ppm Ca and 2.3 meq/L alkalinity without spontaneous precipitation — and what lets your tank hold the same. Below ~1,150 ppm, the inhibition fails and you start fighting an uphill battle to keep Ca and Alk elevated.

Symptoms of drift

Too low

Below 1,150 ppm: uncontrolled Ca/Alk precipitation, snowstorming, corals failing to calcify normally despite "good" Ca and Alk numbers, coralline algae dying back. The classic 'I can't keep my Alk stable' tank is usually a Mg deficiency.

Too high

Above 1,500 ppm: elevated sulfate (if dosed with Epsom salts), generally non-toxic up to ~1,800 ppm but provides no benefit. Test kit titrations get harder to read above 1,500.

Testing

Salifert and Red Sea Magnesium Pro titration kits work fine — Red Sea reports 20 ppm resolution, real-world ±50 ppm. Test monthly unless you've recently dosed. ICP labs (ATI, Triton) are worth running every few months for a sanity check. Hanna doesn't make a colorimeter-style Mg checker; the chemistry doesn't translate.

Dose with Randy Recipe #3 (MgCl₂ + Epsom blend) or Red Sea Foundation C for ongoing maintenance. Pure Epsom salt works for a one-time correction but skews sulfate over months.

FAQ

Why does my Mg keep dropping?
Coral and coralline incorporate Mg into their CaCO₃ skeletons (about 4 mol% of calcite). Water changes only replace what's in the salt mix. A 10 % change only restores 10 % of the gap between your tank and the salt mix: if you consume 30 ppm/week and the tank sits 30 ppm below the mix, a weekly 10 % change returns about 3 ppm — a tenth of your loss. Dose proactively.
Can I dose Epsom salt only?
For a one-time correction, yes. Ongoing use builds sulfate over months and skews salt ratios. Switch to a blend (Randy Recipe #3) or Red Sea Foundation C for daily maintenance.

Test kits & supplies

Affiliate links

Salifert magnesium test kit is the reef-hobby standard. Test monthly \u2014 Mg drift is the silent cause of Ca and alk crashes. When it tests low, a chloride + sulfate blend (BRS Pharma Magnesium or Red Sea Foundation C) brings it back.

Salifert Magnesium test kitTest monthly. Mg drift is the silent cause of Ca/alk drops.View on Amazon →BRS Pharma Magnesium MixMagnesium chloride + sulfate blend for Randy Recipe 3 magnesium.View on Amazon →Red Sea Reef Foundation C (Magnesium) — 1 kg powderMagnesium component. Dose only when Mg tests low.View on Amazon →

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

Sources & references

  1. 1.
    Randy Holmes-Farley — Optimal Parameters
    https://www.reef2reef.com/threads/optimal-parameters-for-a-coral-reef-aquarium-by-randy-holmes-farley.173563/
  2. 2.
    Randy Holmes-Farley — Magnesium in Reef Aquaria
    https://reefs.com/magazine/aquarium-chemistry-magnesium-in-reef-aquaria/
  3. 3.
    Randy Holmes-Farley — DIY Magnesium Supplements, Recipe #3 (Reefkeeping)
    https://reefkeeping.com/issues/2006-07/rhf/index.php
  4. 4.
    Triton — CoreChem
    https://www.triton.de/en/products/corechem
  5. 5.
    Global Seafood Alliance — Typical chemical characteristics of seawater
    https://www.globalseafood.org/advocate/typical-chemical-characteristics-of-full-strength-seawater/

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