Alkalinity, Calcium, and Magnesium

Every reef keeper learns the three numbers. Far fewer learn that they are not three independent parameters you tune separately, but one coupled chemical system where moving any of them changes the others.

If you want the target ranges and what each parameter means on its own, the reef parameters guide covers all of them including salinity, nitrate, and phosphate. This guide is about why these particular three move together, what their relationship predicts, and how to correct one without wrecking another.

What alkalinity actually measures

Alkalinity is not pH, and confusing the two causes a lot of unnecessary dosing. pH tells you where the water currently sits. Alkalinity tells you how hard it is to move, since it measures the water's capacity to neutralise acid. In seawater that capacity is mostly bicarbonate and carbonate, which are the same ions corals use to build skeleton. That dual role is the whole story: your buffer and your building material are the same pool.

The hobby measures it in three different units, which is a recurring source of confusion:

For reference, natural seawater runs about 2.5 meq/L, which is roughly 7 dKH, with calcium near 420 ppm and magnesium near 1280 ppm.

Why the three move together

Coral skeleton is calcium carbonate, deposited as aragonite. The reaction that builds it is:

Ca²⁺ + 2 HCO₃⁻ → CaCO₃ + CO₂ + H₂O

Read the left side carefully. Every unit of skeleton laid down removes one calcium ion and two units of alkalinity. That fixed ratio is not a rule of thumb, it is stoichiometry, and it gives you an exchange rate you can rely on:

• 1 meq/L of alkalinity consumed corresponds to about 20 ppm of calcium consumed.
• 1 dKH of alkalinity consumed corresponds to about 7.1 ppm of calcium consumed.

Nothing in a healthy reef tank consumes alkalinity without consuming calcium in that proportion, which makes the relationship a diagnostic tool. If your alkalinity is falling and calcium is falling proportionally, that is coral growth. If alkalinity is falling and calcium is not moving at all over a long period, something other than calcification is eating your buffer.

Why your alkalinity drops and your calcium looks fine

This is the question that confuses almost every new reefer, and the answer is not chemistry at all. It is reservoir size.

Take a tank at 8 dKH and 420 ppm calcium. Corals consume enough to drop alkalinity by 1 dKH. Proportionally, that same growth consumed about 7 ppm of calcium. So:

• Alkalinity fell by 1 of 8, which is 12.5% of the pool, and every test kit on the market will see it.
• Calcium fell by 7 of 420, which is 1.7% of the pool, and that is comfortably inside the margin of error of a typical hobby calcium kit.

Both parameters were consumed in exact proportion. Only one of them visibly moved. This is why alkalinity is your early-warning parameter and the one worth testing most often: it is simply the most sensitive readout of what your corals are doing. Calcium is the slow, stable one, and a calcium level that looks rock-steady week after week is telling you very little.

It is also why chasing calcium is usually wasted effort. In a tank with healthy calcium, dosing more of it does not increase growth. Alkalinity is the parameter that actually runs out first.

Magnesium, and why the other two can coexist at all

Here is the part that is genuinely counterintuitive. At reef calcium and alkalinity levels, seawater is supersaturated with respect to calcium carbonate. Chemically it wants to precipitate. Left to pure thermodynamics, your calcium and carbonate should be falling out of solution onto every surface in the tank.

Magnesium is what stops it. Magnesium ions are similar enough to calcium to adsorb onto the growing faces of a calcium carbonate crystal, but different enough that they block further growth rather than joining it. The result is that abiotic precipitation is poisoned before it can run away, while corals, which build under biological control, carry on regardless.

The practical consequence is direct: below roughly 1250 ppm of magnesium, you cannot hold calcium and alkalinity up at the same time. Dose alkalinity and it precipitates. Dose calcium and it precipitates. You see cloudy water after dosing, white dust accumulating on heaters and pump impellers, and test results that stubbornly refuse to move no matter how much supplement goes in. Keepers in this situation usually conclude their supplement is weak. It is not; the magnesium is.

Magnesium is consumed too, partly into coral skeleton and partly by any precipitation event, but it is a very large reservoir and it moves slowly. Test it monthly rather than weekly.

Targets, and why stability beats the number

These are the ranges the reef dosing calculator uses, and they are ordinary hobby consensus rather than anything proprietary:

Two things about that table matter more than the numbers themselves.

First, natural seawater alkalinity is about 7 dKH, below the bottom of the "acceptable" range. The reef range is a husbandry target chosen to give you buffer against consumption between doses, not a natural baseline you are restoring. A tank running steadily at 7.5 dKH is not deficient.

Second, and more important: corals respond to change, not to absolute values. A reef held rock-steady at 8 dKH will consistently outperform one sawtoothing between 8 and 11, even though the second tank spends much of its time at "better" numbers. Rapid alkalinity swings are one of the most common causes of tissue recession and burnt tips in SPS. If you have to choose between hitting your ideal number and holding your current one steady, hold it steady.

Correcting safely

Order matters, and it follows directly from the chemistry above.

1. Magnesium first. If magnesium is low, alkalinity and calcium corrections will not hold, because the excess precipitates instead of staying in solution. Fixing the other two first is wasted supplement.
2. Alkalinity second, at no more than about 1 dKH per day. This is the one that hurts livestock when rushed.
3. Calcium last. It is the most forgiving of the three and the least likely to be genuinely out of range once the other two are right.

There is one hard rule alongside that order: never dose calcium and alkalinity into the same place at the same time. Where the two streams meet you create, briefly and locally, exactly the supersaturation that magnesium is working to prevent. Part of both doses precipitates instead of dissolving. Separate them by a few hours, or add them to different high-flow areas of the sump. Two-part dosing pumps stagger their channels for precisely this reason.

Keeping up with consumption

Once you know your daily consumption, the method follows from its size. Measuring it is simple: test at the same time on two consecutive days without dosing in between, and the difference is your daily uptake.

• Water changes alone work only for very low demand, typically a soft coral or fish-only system.
• Two-part dosing adds calcium and alkalinity from separate bottles in the balanced ratio. It suits most small to medium reefs and scales cleanly with a doser.
• Kalkwasser (limewater) adds both in balance through your top-off water, and raises pH as a side effect, which is often welcome in a tank running high CO₂. Its ceiling is set by how much top-off the tank evaporates.
• A calcium reactor dissolves aragonite media with CO₂ and suits high-demand SPS systems. More equipment, more stability, more to go wrong.

Whatever you choose, dose against a measured consumption rate rather than a schedule copied from someone else's tank. Demand scales with how much coral you have and how fast it is growing, so it changes over the life of the system.

What a falling number is telling you

The most useful reframe in reef chemistry is that consumption is not a problem to be solved. A tank whose alkalinity drops steadily between tests is a tank that is building skeleton. A tank whose alkalinity does not move at all is either very lightly stocked or not growing, and the second is worth investigating.

So track the rate, not just the value. A consumption rate that suddenly rises usually means growth, and one that suddenly falls often precedes a visible problem by days. The coral placement guide covers the light and flow side of the same question, and the reef parameters guide puts these three back in context with salinity, nitrate, and phosphate.

Frequently Asked Questions

Why does my alkalinity keep dropping when my calcium barely moves?

Because the two reservoirs are wildly different sizes. Building coral skeleton removes calcium and alkalinity in a fixed ratio: about 7.1 ppm of calcium for every 1 dKH of alkalinity. But 1 dKH is over 12% of an 8 dKH tank, while 7 ppm is under 2% of a 420 ppm calcium level, and it sits inside most hobby calcium kits' margin of error. Falling alkalinity with steady calcium is the normal signature of a growing reef, not a problem.

What does magnesium actually do?

It stops your calcium and alkalinity from precipitating out of the water. At reef levels, seawater is supersaturated with respect to calcium carbonate, meaning it chemically wants to form solid CaCO₃. Magnesium ions adsorb onto growing crystal faces and block them, which keeps the water stable. Below roughly 1250 ppm you cannot hold calcium and alkalinity up at the same time: dosing either one tends to precipitate instead, often visible as white dust on heaters and pump impellers.

What order should I correct the big three in?

Magnesium first, then alkalinity, then calcium. Correcting alkalinity or calcium while magnesium is low usually will not hold, because the excess precipitates. Once magnesium is in range, bring alkalinity up slowly, and treat calcium last since it is the most forgiving of the three.

How fast can I safely raise alkalinity?

No more than about 1 dKH per day. Calcium tolerates about 20 ppm per day and magnesium about 50 ppm per day. Those limits exist because corals respond to the rate of change, not just the destination: a fast alkalinity correction is a common cause of tissue recession and burnt tips in SPS, even when the final number is perfectly reasonable.

Can I dose calcium and alkalinity at the same time?

Not into the same spot at the same time. Where the two streams meet you briefly create exactly the local supersaturation that makes calcium carbonate precipitate, so some of both doses ends up as dust rather than in solution. Separate them by a few hours, or dose them into different high-flow areas of the sump. Automated two-part dosers stagger them for this reason.

Is a higher alkalinity better for coral growth?

Only up to a point, and stability matters more than the number. Corals adapted to a steady 8 dKH do better than corals riding a sawtooth between 8 and 11, and higher alkalinity is less forgiving of low nutrients and swings. Natural seawater sits around 7 dKH, so the usual reef target of 8 to 9.5 dKH is already a husbandry choice rather than a natural baseline.