Parameter Reference
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-safe | 1,200–1,400 ppm |
| Optimal (consensus) | 1,250–1,350 ppm |
| Triton method target | 1,300–1,380 mg/L |
| Natural seawater | ~1,262–1,290 ppm |
| Randy's NSW figure | ~1,285 ppm (53 mM) |
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 ~410 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 3A (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. If you consume 30 ppm/week and change 10 % weekly, your replenishment is roughly 130 ppm × 0.1 = 13 ppm — half 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 (Recipe 3A) or Red Sea Foundation C for daily maintenance.
Sources & references
- 01Randy Holmes-Farley — Optimal Parametershttps://www.reef2reef.com/threads/optimal-parameters-for-a-coral-reef-aquarium-by-randy-holmes-farley.173563/
- 02Randy Holmes-Farley — Magnesium in Reef Aquariahttps://reefs.com/magazine/aquarium-chemistry-magnesium-in-reef-aquaria/
- 03Randy Holmes-Farley — Reefkeeping Recipe 3A (Mg)https://reefkeeping.com/issues/2006-07/rhf/index.php
- 04Triton — CoreChemhttps://www.triton.de/en/products/corechem
- 05Global Seafood Alliance — Typical chemical characteristics of seawaterhttps://www.globalseafood.org/advocate/typical-chemical-characteristics-of-full-strength-seawater/
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