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Magnesium — the invisible stability mechanism

Basics Published: 2 June 2026

The third element, a different role

Alkalinity and calcium are familiar to most hobbyists: they are the raw materials of coral skeletons, measured regularly and their consumption shows directly. Magnesium is the third element in this group, but its role is different — and it does not receive the attention it deserves.

Magnesium is not really a skeleton-building material in the same sense as calcium. Its job is to keep the calcium carbonate system functioning. Without adequate magnesium, calcium carbonate chemistry becomes unpredictable — and then maintaining calcium and alkalinity becomes difficult regardless of how well they are dosed.

Why seawater contains so much magnesium

Natural seawater contains about 1,290–1,350 mg/l of magnesium. Calcium is 410–420 mg/l. In molar terms this means roughly three times as much magnesium — far more than one might intuitively expect.

This ratio is not coincidental. It reflects magnesium’s central chemical role in seawater: preventing uncontrolled crystallisation of calcium carbonate.

Seawater is saturated with calcium carbonate. From an equilibrium standpoint, CaCO₃ should continuously precipitate onto surfaces and leave solution. This does not happen — and one of the most important reasons is magnesium. Mg²⁺ ions disrupt calcite crystal growth so effectively that spontaneous precipitation is prevented. Biological mechanisms can then use calcium in a controlled manner.

Magnesium and aragonite — why it matters mineralogically

Stony corals build their skeletons from aragonite, one of the two common crystal forms of calcium carbonate. The other form is calcite. Thermodynamically, calcite would be more stable under seawater conditions — yet corals build aragonite.

The reason is magnesium.

Mg²⁺ ions fit well into the Ca²⁺ positions of the calcite crystal lattice but poorly into the corresponding positions of the aragonite lattice. When magnesium is sufficient, it pushes into the surface of a growing calcite crystal and disrupts crystal growth so forcefully that calcite becomes unstable. In practice, magnesium closes the calcite pathway — and the coral uses aragonite, for which its biology is optimised.

This means that insufficient magnesium does not just cause “less magnesium in the skeleton”. It can alter the mineralogy of calcification and weaken the skeleton’s structure in a way that does not show up in any single measurement.

Magnesium keeps calcium dissolved

The other key effect is more practical and shows directly in tank maintenance.

Mg²⁺ and Ca²⁺ ions compete for crystallisation sites on surfaces. When magnesium is sufficient, it covers active crystallisation sites and prevents spontaneous precipitation of calcium carbonate onto equipment, pipework and rock surfaces. This keeps calcium dissolved in the water — where the coral can use it.

When magnesium is too low, this mechanism weakens. Calcium begins precipitating spontaneously: heaters accumulate calcium scale, pumps make grinding sounds, pipework develops deposits. These are the physical signs of a magnesium problem — not just equipment malfunction symptoms.

Magnesium consumption in the tank

Magnesium is consumed primarily by coralline algae (especially CCA) and stony corals, which incorporate small amounts into their skeletons. Consumption is noticeably slower than calcium or alkalinity — which is why magnesium is measured less frequently than the other two.

In some tanks with abundant coralline algae growth or high consumption from another source, magnesium can drop surprisingly quickly. The C component of the Balling method contains magnesium, but it is sized to compensate for ionic balance — not necessarily for large biological consumption. Heavy CCA growth or a vigorously growing coral mass may require separate magnesium supplementation.

References

1. Peer-reviewed studies

2. Hobbyist literature and brand documentation

3. Books and textbooks