Calcium, KH and magnesium in a reef: the triangle

Updated on August 26, 2026 · ~6 min read

In a reef you hold calcium at 400-450 ppm, KH at 7-9 dKH and magnesium at 1250-1400 ppm (about three times the calcium). The three are linked: without magnesium in order, calcium and alkalinity precipitate instead of staying dissolved, so if they will not come into range the first thing to check is magnesium.

Hard corals build their skeleton out of calcium carbonate. To do it they consume calcium and alkalinity from the water, in the same proportion in which they lay them into the skeleton — so in a growing reef the two values fall together, every day, and have to be replenished.

Magnesium does not enter the skeleton in any significant quantity, but it decides whether the other two can stay dissolved. That is why we talk about a triangle and not three separate parameters.

Reference values

Parameter	Reef	Notes
Calcium	400-450 ppm	not measured in freshwater
Alkalinity (KH)	7-9 dKH	and above all stable
Magnesium	1250-1400 ppm	about 3 times the calcium

The "magnesium ≈ 3 × calcium" ratio is the quickest way to check by eye whether the triangle is coherent: 420 ppm of calcium wants about 1260-1350 ppm of magnesium. Magnesium at 1000 with calcium at 430 is the combination that half of all dosing problems come from.

Why they are linked

In sea water, calcium and carbonates sit close to the solubility limit: there is almost more of them than the water can hold dissolved. Magnesium interferes with the formation of calcium carbonate crystals and stops them as they form — it is, in practice, what keeps the two from precipitating onto each other.

Three behaviours that surprise beginners follow from this.

If magnesium is low, calcium and KH will not rise. You dose, you measure, and the value is where it was: what you added has precipitated — often as a whitish film on the heater, the pumps and the glass. Carrying on dosing makes things worse. Magnesium first, then the other two.

Raising KH abruptly makes calcium precipitate. In a tank with low KH, dosing buffer all at once locally exceeds solubility: the water clouds for a few hours (a "snowstorm") and calcium falls instead of staying up. That is why buffer is dosed diluted, slowly and near a pump.

High consumption = growth. In a reef with SPS the daily consumption of calcium and alkalinity is the measure of how much the tank is growing. Dosing is calibrated on that consumption, not on a dose recommended on the box.

Why KH matters more than the other two

Alkalinity is the reserve of carbonates and bicarbonates: it is also what buffers pH. From which the most important rule on this page follows:

A practical consequence also follows: KH is the one of the three to measure most often. In a stocked reef it is worth looking at once or twice a week; calcium and magnesium move more slowly and tolerate a check every one to two weeks.

How they are dosed

The Balling (or two-part) method. Two separate solutions — one calcium, one alkalinity — dosed in equal amounts, plus magnesium separately when needed. They must be added at different points in the tank or at different times: together and in the same spot they precipitate with each other before they dilute. It is the most widespread system and the most controllable.

A calcium reactor. It dissolves calcium carbonate with CO₂ and replenishes calcium and alkalinity in the right proportion, automatically. It costs more up front and needs tuning, but on a large tank it is cheaper and more stable.

Kalkwasser (calcium hydroxide). Limewater used as top-off: it replenishes both and raises pH, which in many closed rooms is low because of the room's CO₂. It is not enough on tanks with high consumption and must be dosed cautiously, because its pH is very high.

Magnesium is dosed separately, with magnesium chloride and sulphate, and slowly: spread the correction over several days, because there is little to gain by rushing and a jump is not better.

For small adjustments in freshwater — and to get a sense of the order of magnitude — it is worth knowing that about 3 g of baking soda per 100 litres (26 gal) raises KH by 1 dKH. In a marine tank baking soda on its own is an emergency measure, not a system: it raises KH without touching calcium, so used over time it unbalances the triangle.

How to measure them, and where it goes wrong

Calcium, KH and magnesium are measured with syringe titrations: the result is read on the syringe scale, not on the colour. Which means a tenth of a millilitre is tens of ppm of calcium, and that all the precision is in your hand.

Three typical errors:

• An approximate sample volume. As with GH and KH in freshwater, the count depends on the exact volume. The mark, not "nearly".
• The colour change read late. Add drop by drop, slowing down near the change: rushing overshoots it and reads a value that is too high.
• Comparing two brands. Between two calcium kits there can be a systematic difference of 20-30 ppm. To follow consumption what matters is using the same one every time: the variation between two readings is the information, not the absolute value.

The order in which you fix an unbalanced tank

1. Magnesium. If it is low, nothing else moves. Bring it into range, slowly.
2. Alkalinity. In small steps, at most about 1 dKH per day.
3. Calcium. It nearly follows on its own, and in any case it moves more slowly.
4. Only then do you calibrate the daily dose on measured consumption.

And before all of it: a water change. A quality salt brings the three values close to the references, so in a badly unbalanced tank two substantial changes do half the work without dosing anything.

How Gurglee does it

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