Reef Tank Magnesium: Target Range, Why It Matters, Testing, and Dosing

Magnesium is the silent enabler of reef chemistry. It does not visibly do much until it drops too low — then suddenly calcium and alkalinity stop responding to dosing. Most reef keepers only discover magnesium when they hit that wall.

Reef Chemistry March 18, 2026 By Joe Marino 2 min read

What magnesium does in a reef tank

Magnesium prevents calcium and alkalinity from precipitating out of solution together. Calcium plus carbonate naturally wants to combine into calcium carbonate crystals; magnesium ions interfere with that crystallization, keeping calcium and alkalinity dissolved and available to corals.

When magnesium drops below roughly 1250 ppm, calcium and alkalinity start precipitating directly out of the water before corals can use them. The symptom: you dose calcium and alkalinity but neither rises (or both fall). The fix: raise magnesium to 1300+ first, then resume calcium and alkalinity dosing.

Target magnesium range

Natural seawater is about 1280 ppm. Most reef keepers target 1300 to 1400 ppm in a reef tank for a small buffer above natural seawater. Some Berlin-style reef keepers run as high as 1500 ppm.

Natural seawater: ~1280 ppm
Standard reef target: 1300 to 1400 ppm
Acceptable range: 1250 to 1500 ppm
Below 1250 ppm: calcium and alkalinity dosing stops working
Above 1500 ppm: no real benefit; can occasionally interfere with iodine and other trace elements

How magnesium is consumed

Magnesium is consumed much more slowly than calcium or alkalinity. Most reef tanks see magnesium move 10 to 30 ppm per month. Coralline algae growth consumes magnesium; SPS corals consume small amounts. Skipped water changes are the most common cause of long-term magnesium decline.

How to test magnesium

Magnesium tests are titrations and can be slightly tedious. Recommended:

Salifert Magnesium Test — accurate, widely available, easy to read
Red Sea Foundation C Test — pairs with Foundation A (Ca) and B (Alk)
Hanna HI783 Magnesium Checker — colorimeter (less common but available)
ICP results from Aquaforest, ATI, Fauna Marin, Triton, and others — most accurate

How to raise magnesium

Two-part C (magnesium chloride + magnesium sulfate blend) is the standard. Brands: BRS, Red Sea Foundation C, Tropic Marin Bio-Magnesium, ESV.

Math: 1 mL of standard concentration 2-part C in 100 L raises magnesium by roughly 1 to 2 ppm depending on the product. Reef Trak's dosing calculator has the per-product math built in. Magnesium raises faster than calcium because the dose has more headroom; doses of 50 ppm per day are tolerated without issue.

How to lower magnesium

Stop dosing, do water changes with a lower-magnesium salt mix. There is no inverse dose for magnesium and rarely a real need to lower it.

Frequently asked

What is the ideal magnesium for a reef tank?

Most reefers target 1300 to 1400 ppm. Natural seawater is about 1280 ppm. Below 1250 ppm causes calcium and alkalinity dosing to stop working.

Why is my calcium dosing not working?

Low magnesium is the most common cause. When magnesium drops below ~1250 ppm, calcium and alkalinity precipitate out of solution before corals can use them. Test magnesium first; raise to 1300+ before continuing calcium dosing.

How fast does magnesium move in a reef tank?

Slowly. Most reef tanks move 10 to 30 ppm per month. Coralline algae growth is the largest steady consumer.

How do I raise magnesium safely?

Two-part C dosing (magnesium chloride + sulfate blend) is the standard. Doses of 50 ppm per day are tolerated. Brands include BRS, Red Sea Foundation C, Tropic Marin Bio-Magnesium, ESV.

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