The big three in practice — measurement, target values and balance

One system, three measurement points

Hobbyists often talk about alkalinity, calcium and magnesium as three separate parameters — three different things tracked and adjusted independently. This is a practical way to think about it, but it is also somewhat misleading.

These three elements form a single chemical system, measured at three different points. Alkalinity tells you how much carbonate and bicarbonate ions are in the water. Calcium tells you how much Ca²⁺ is available. Magnesium keeps the whole balance in check by preventing uncontrolled calcium carbonate precipitation. Change one, and the other two inevitably move.

This is the article’s single most important insight, and it changes how measurements should be interpreted.

Why these three are linked

When a coral builds its skeleton, it takes Ca²⁺ ions and carbonate ions (CO₃²⁻) from the water and combines them into calcium carbonate (CaCO₃), which crystallises in aragonite form. The process consumes both simultaneously — not just one.

This is where alkalinity comes in. Alkalinity is the water’s acid-binding capacity, and its most important component in a reef is bicarbonate ions (HCO₃⁻) and carbonate ions (CO₃²⁻). They act as a buffer that keeps pH stable, and they are simultaneously the raw material for calcification. When alkalinity falls, calcium carbonate formation slows — even if enough calcium is present.

Magnesium’s role is more subtle. Mg²⁺ ions compete with Ca²⁺ ions for crystallisation sites in carbonate minerals. When magnesium occupies calcite crystal structures in place of Ca²⁺, it makes the crystal less stable and more soluble. The practical consequence is significant: sufficient magnesium keeps calcium dissolved in the water and prevents its spontaneous precipitation onto equipment, pipes and rock surfaces before the coral can use it.

A stable calcium value is not possible without sufficient magnesium. This is not a metaphor — it is chemistry.

Reference values and target values: where should you be?

Natural seawater as a reference

In natural seawater (NSW) the three main element values vary by region and season. Typical reference values:

The molar ratio of calcium to magnesium in seawater is approximately 1:3 — three times more magnesium than calcium.

These are reference values, not target values. An aquarium is not the open ocean. In a closed system, consumption is proportionally much greater, and small deviations appear more acutely because the water volume is limited.

Target values by tank type

More important than where in the range you are is whether you stay there. Trend is always more important than a single reading — this is covered in depth in article #6 The stability principle.

Mutual balance: which combination is unstable?

The calcium carbonate saturation state (Ω) is determined by the product of Ca²⁺ and CO₃²⁻ ion concentrations relative to the solubility product of calcium carbonate. When Ω exceeds 1, the solution is supersaturated and spontaneous CaCO₃ precipitation becomes possible. In seawater Ω is typically around 3 (for aragonite), which keeps reefs functional without uncontrolled crystallisation — thanks to biological and chemical factors, magnesium among them.

In an aquarium the problem arises when both alkalinity and calcium are raised well above natural values simultaneously. Magnesium suppresses this crystallisation tendency, but the effect is limited if supersaturation is large enough.

Practical warning signs of spontaneous precipitation:

• White precipitate on pump impellers or on the heater surface
• Chalky deposits in plumbing and on tank rims
• Momentary clouding near dosing points (“snow shower”)

If alkalinity is well above 11–12 dKH and calcium simultaneously above 470 mg/l, precipitation risk is real. Do not drive target values simultaneously far beyond NSW in both directions.

Measurement: methods, accuracy and rhythm

Alkalinity

Alkalinity is the easiest of the three to measure at home, and test equipment is reliable.

Titrimetric tests:

• Salifert KH test — simple titrimetry, one drop = 0.1 dKH, practical and reliable for weekly measurement
• Red Sea KH Pro — wider scale, colour interpretation requires good light
• Hanna HI-772 Checker — photometer, digital reading, less subjectivity than colour-change tests

Laboratory-grade automatic titrimetry is more accurate than home tests. Alkalinity is included in all ICP laboratory packages, and it is worth comparing against your own home test a few times a year — systematic differences between reagent batches are possible.

Calcium

Home tests are titrimetric and fully adequate for routine monitoring:

• Salifert Ca test — accuracy ~5–10 mg/l, reliable
• Red Sea Calcium Pro — slightly better accuracy at lower concentrations

An ICP laboratory is the only way to check calcium, other macro-elements and trace elements simultaneously in a single sample.

Magnesium

Home tests are titrimetric like calcium. Accuracy is weaker — typically ±10–20 mg/l — which is sufficient for trend monitoring but not precise diagnosis. An ICP laboratory is practically the only way to get magnesium reliably, especially when consumption is unclear.

• Salifert Mg test — best home test for magnesium on the European market

Measurement rhythm

ICP laboratory every 4–6 weeks in an established tank. A more frequent rhythm is warranted only if something has clearly changed or the dosing amount is being significantly adjusted.

Signs of imbalance

The imbalance of the three main elements does not always manifest immediately or dramatically. Often the first signs are subtle.

Alkalinity too low (< 6 dKH):

• Slowing of coral growth
• Polyp retraction, particularly in branching Acropora species
• Overnight pH drop intensifies — insufficient buffer capacity worsens night-time pH decline

Alkalinity too high (> 12 dKH) combined with high calcium:

• White precipitate on equipment
• Coral stress response — polyps do not open normally
• In acute cases STN/RTN reaction

Calcium too low (< 350 mg/l):

• Calcification slows noticeably
• Coral skeleton thins over time
• Euphyllia species often react first — skeleton begins to show through tissue

Magnesium too low (< 1,100 mg/l):

• Alkalinity and calcium begin fluctuating inexplicably even though dosing does not change
• Coralline algae growth slows or stops
• LPS coral pigmentation weakens

Mutual imbalance:

If all three are simultaneously low, the cause is usually high consumption (growth phase, abundant corals) or insufficient supplementation. If calcium falls but alkalinity stays stable, coralline algae is a common culprit especially during growth phases.

An aquarium is not the natural ocean — why slightly above NSW is justified

In the natural reef, corals live in a vast water mass that buffers changes effectively. In an aquarium, corals consume a proportionally many times larger share of available ions in the same time. This means values are more susceptible to fluctuation — both too rapid a decline and too rapid a rise are risks.

Running slightly above NSW for alkalinity provides a buffer against daily consumption without yet being in the precipitation risk zone. More important than the level chosen is whether the chosen level is maintained. Practical advice: choose a target, maintain it for a month, run ICP, check the trend. Change one thing at a time.

Summary

1. Alkalinity, calcium and magnesium are one system — change one and the other two move
2. Choose the appropriate target range for your tank type, do not chase maximum values
3. Measure alkalinity weekly — it changes fastest of these three
4. Check calcium every two weeks, magnesium once a month
5. ICP every 4–6 weeks keeps data interpretable
6. Follow the trend, do not react to a single measurement
7. If all three are low, the primary cause is consumption — not a dosing error
8. If alkalinity fluctuates but calcium does not, the problem is usually magnesium

Dosing methods — kalkwasser, two-part, Balling, ion-based two-part, reactor — will be covered in the next article #20. The important thing in this article is understanding what is being maintained and why, before deciding with what.

References

1. Peer-reviewed studies

• Tambutté, S. et al. (2011). Coral biomineralization: From the gene to the environment. Journal of Experimental Marine Biology and Ecology, 408(1–2), 58–78. https://doi.org/10.1016/j.jembe.2011.07.026
• Holcomb, M. et al. (2010). Coral calcification regulated by kinetics and not saturation state. Geochimica et Cosmochimica Acta, 74(17), 4926–4938.
• Venn, A. et al. (2019). Impact of seawater acidification on pH at the tissue–skeleton interface and calcification in Stylophora pistillata. PNAS, 116(36), 17648–17655.
• Comeau, S. et al. (2022). pH variability at reef scales is a biological amplifier of coral calcification responses. PLOS ONE.
• McCulloch, M. et al. (2012). Resilience of cold-water scleractinian corals to ocean acidification. Geochimica et Cosmochimica Acta, 87, 21–34.

2. Hobbyist literature and industry documentation

• Fauna Marin (2024). Alkalinity / Carbonate Hardness — Knowledge Base. https://www.faunamarin.de/en/alkalinity-carbonate-hardness/
• Fauna Marin (2024). Calcium — Knowledge Base. https://www.faunamarin.de/en/knowledge-base/calcium/
• Fauna Marin (2024). Magnesium — Knowledge Base. https://www.faunamarin.de/en/knowledge-base/magnesium/
• Reef Zlements (2024). Hybrid 2-Part Dosing System — System Manual. www.zlements.com
• Aslett, C.G. (2024). SPS Academy Part V — Teach a Person to Fish. reefranch.co.uk
• Holmes-Farley, R. (2002). Chemistry and the Aquarium article series. Advanced Aquarist.

3. Books and textbooks

• Borneman, E.H. (2001). Aquarium Corals: Selection, Husbandry, and Natural History. Microcosm. ISBN 1-890087-47-5.
• Dickson, A.G. (2023). Alkalinity in theory and practice. Elements, 19(1), 7–12.