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Magnesium in practice — measurement, target values and dosing

Practice Published: 2 June 2026

The underrated parameter

Magnesium is noticeably less monitored among hobbyists than alkalinity or calcium. The reason is understandable: it depletes more slowly, changes do not show as quickly, and home test accuracy is more limited. Often magnesium is only checked when something else is not working — when alkalinity fluctuates inexplicably, when calcium disappears onto equipment, or when growth has stopped without an obvious cause.

This is a mistake. Magnesium is the calcium carbonate system’s stability mechanism, and it is worth measuring proactively — not only after a problem has appeared.

Target values

Natural seawater magnesium is 1,290–1,350 mg/l. In a reef aquarium, the recommended range is slightly wider:

Tank typeMagnesium (mg/l)
Softie / LPS tank1,250–1,350
Mixed reef1,280–1,380
SPS-dominant1,300–1,400

Below 1,200 mg/l becomes a clear risk zone: the mechanism preventing spontaneous calcium precipitation weakens and calcification mineralogy can be disrupted. Above 1,500 mg/l is toxic to some invertebrates — in practice this is not reached with normal supplementation dosing, but it is worth knowing if magnesium is supplemented all at once in large doses.

The molar ratio of calcium to magnesium in natural seawater is about 1:3. This ratio is worth keeping in mind when reviewing ICP results: if calcium is significantly high relative to magnesium, precipitation risk increases.

Measurement methods

Home tests

Home magnesium measurement is titrimetric like calcium — but accuracy is weaker. A typical home test gives a result with ±20–30 mg/l accuracy, sufficient for trend monitoring but not for diagnosing a precise situation.

Home test usefulness: adequate for checking whether magnesium is “in the right range” or clearly off. Not sufficient for detecting a subtle gradual decline — for that, ICP is needed.

ICP laboratory

ICP is practically the only way to monitor magnesium reliably. Home test scatter of ±20–30 mg/l means a 100 mg/l drop can only be detected once it is obvious — ICP detects the change much earlier.

ICP also shows magnesium in relation to other elements: it needs to be seen relative to calcium, sulphate and sodium, not just as an absolute value.

Recommended schedule: ICP every 4–6 weeks always includes magnesium. Home test once a month for trend monitoring between ICP runs.

Why magnesium falls

Magnesium consumption is usually slow, but it can fall for several different reasons:

Biological consumption — stony corals incorporate small amounts of magnesium into their skeletons; coralline algae (CCA) consume it actively. With abundant coralline algae coverage, consumption can be surprisingly large.

Dosing method limitation — not all dosing methods replenish magnesium automatically or adequately. The C component of the Balling method contains magnesium, but is sized for ionic balance compensation — not necessarily for large biological consumption. Two-part products contain magnesium in the calcium component, but the quantity may be insufficient in vigorously growing tanks.

Magnesium salt impurities — magnesium sulphate and magnesium chloride used in supplementation can contain impurities. ICP reveals these — for example, a bromide rise is a classic sign of impure magnesium chloride.

Absence of water changes — a long period without water changes combined with strong biological consumption leads to gradual decline.

Dosing and correction

Proactive maintenance

For most tanks, normal dosing system operation (two-part, Balling, ionic) is sufficient to maintain magnesium. The ICP result tells you whether maintenance dosing is adequate or whether a separate magnesium component needs to be added.

Important exception: the original Balling method, kalkwasser and CO₂ reactor contain no magnesium at all — they replenish only calcium and alkalinity. When using these methods, a separate magnesium supplement is a mandatory part of maintenance, not an optional extra. Without it, magnesium gradually falls and the calcium system begins to destabilise over weeks or months.

Separate magnesium supplements (magnesium sulphate or magnesium chloride) are the simplest way to replenish magnesium when needed. Sulphate form also adds sulphate — which is also needed — and is therefore often the first choice if sulphate is not already elevated.

Raising from a low level

If magnesium has fallen clearly below target, the rise is done slowly — a maximum of 50–100 mg/l per day. Too rapid a rise can disturb ionic balance and stress corals.

Practical example: magnesium 1,100 mg/l → target 1,300 mg/l. Difference 200 mg/l, raise over 2–4 days in small increments. Measure before each addition.

Do not raise magnesium all at once with a large dose. Unlike calcium or alkalinity, a magnesium overdose is harder to detect quickly and can accumulate.

Signs of problems

Too-low magnesium (< 1,200 mg/l):

Signs of impure magnesium salts:

Too-high magnesium (> 1,500 mg/l):

Practical checklist

  1. Measure magnesium with a home test once a month — ICP every cycle
  2. Check calcium-to-magnesium ratio in ICP, not just absolute values
  3. If calcium is precipitating onto equipment, check magnesium before anything else
  4. Raise magnesium slowly — maximum 50–100 mg/l per day
  5. Use magnesium sulphate first; magnesium chloride if sulphate is already elevated
  6. If your dosing system contains no magnesium component, monitor ICP results closely
  7. Before magnesium supplementation: verify salt purity — check ICP for side effects

References

1. Peer-reviewed studies

2. Hobbyist literature and brand documentation

3. Books and textbooks