Alkalinity, Calcium and Magnesium in Reef Tanks: How They Interact and How to Dose

By the Coral Compass team · Last reviewed 2026-10-15

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Why the Big 3 matter

Stony corals, coralline algae, clams and many other reef organisms build skeletons and shells out of calcium carbonate (CaCO₃). Every bit of skeleton they lay down pulls calcium and carbonate out of the water. In the ocean those ions are replenished endlessly. In a closed aquarium they are not, so a growing reef slowly strips its own water. The three parameters reefers track for this are alkalinity, calcium and magnesium, collectively the Big 3.

Natural seawater sits at roughly 420 ppm calcium, about 1,280–1,300 ppm magnesium and an alkalinity near 2.3–2.5 meq/L (about 6.5–7 dKH). Most reef aquarists run alkalinity a little higher than the ocean, because elevated alkalinity tends to support faster calcification, and run calcium and magnesium close to natural levels.

Common target ranges

Parameter	Natural seawater	Common reef target	Max safe daily change
Alkalinity	~6.5–7 dKH (2.3–2.5 meq/L)	7–9 dKH (many aim for 8–8.5); up to ~11 dKH is workable if stable	~1 dKH per day
Calcium	~420 ppm	400–450 ppm	~50 ppm per day
Magnesium	~1,280–1,300 ppm	1,250–1,400 ppm	~100 ppm per day

These are hobby targets, not laws. The single most consistent piece of advice from experienced reefers and from Randy Holmes-Farley's long-running chemistry articles is that stability matters more than any exact number. A tank that holds 7.5 dKH every day will usually outperform one that bounces between 8 and 10.

Alkalinity: the one to watch most closely

Alkalinity measures the water's capacity to neutralize acid. In seawater it is mostly bicarbonate (HCO₃^-) and carbonate (CO₃^2-), with smaller contributions from borate and others. Corals take up bicarbonate and convert it to carbonate to build skeleton, so alkalinity is usually the fastest-moving of the three in a stony-coral tank.

Units can be confusing. Reef test kits report alkalinity in dKH, meq/L or ppm as CaCO₃. The conversions are simple:

1 meq/L = 2.8 dKH = 50 ppm CaCO₃
1 dKH ≈ 0.357 meq/L ≈ 17.9 ppm CaCO₃

Alkalinity swings are strongly associated with stress in SPS corals, including tissue loss (RTN/STN) and burnt tips. Burnt tips are commonly reported when alkalinity is high and nutrients are very low at the same time. If you only have the budget to test one thing frequently, test alkalinity.

Calcium

Calcium is consumed alongside alkalinity in a fixed ratio set by the chemistry of calcium carbonate. For every 1 meq/L of alkalinity a coral uses, it uses about 20 ppm of calcium. In hobby units that works out to roughly 7.1 ppm of calcium per 1 dKH of alkalinity. That is why a tank that loses 1 dKH a day usually loses about 7 ppm of calcium a day, and why balanced additives such as 2-part and kalkwasser supply both together.

Calcium tends to move slowly and buffers well, so a single low reading is rarely an emergency. A persistently drifting calcium level while alkalinity is stable usually points to an unbalanced additive, a salt mix that runs high or low, or a test error.

Magnesium: the stabilizer

Magnesium is not consumed heavily by most corals, though coralline algae incorporate some into their skeletons. Its main job in reef chemistry is indirect. Magnesium ions bind to the surface of growing calcium carbonate crystals and slow down abiotic precipitation, the process where calcium and carbonate fall out of solution onto heaters, pumps and sand instead of into coral skeletons.

When magnesium runs low, it becomes harder to keep calcium and alkalinity at normal levels at the same time. That is why the classic advice is: if you cannot get calcium or alkalinity to hold, check magnesium first. Magnesium tends to be the slowest of the three to change and is usually tested weekly or less often.

How the three interact

Seawater is supersaturated with calcium carbonate. In other words, there is more calcium and carbonate dissolved than would stay in solution if nothing stopped it. Corals exploit this supersaturation to build skeletons cheaply. Magnesium (and organics and phosphate) keep the water from precipitating on its own.

Push calcium and alkalinity both too high, or let magnesium drop, and you risk a precipitation event: the water turns cloudy, a white crust forms on equipment, and alkalinity and calcium crash together. This is also why you should never pour calcium and alkalinity additives into the same spot at the same time. Add them hours apart into high-flow areas.

A practical diagnostic table:

Alk and Ca both falling at ~1 dKH : 7 ppm: normal coral consumption. Increase balanced dosing.
Alk falling, Ca stable or rising: often an unbalanced additive, or precipitation onto heaters and pumps. Check Mg.
Both falling suddenly and the water is cloudy: precipitation. Stop dosing, do a water change, check Mg.
Readings jumping around with no dosing change: suspect the test. Repeat, check reagent expiry, or cross-check with a second kit.

Measure your daily demand first

Before choosing a dosing method, work out how much your tank actually consumes:

Stop or hold any dosing steady and test alkalinity at the same time each day for 3–5 days.
Average the daily drop. For example, going from 8.4 to 7.6 dKH over two days is 0.4 dKH per day.
Use the Coral Compass dosing calculator to convert that drop into a daily amount of your product for your tank volume.
Dose that amount daily, split across several doses or on a doser, and retest every couple of days. Adjust by no more than about 10% at a time.

Demand is not constant. As corals grow, demand grows, often by a surprising amount over a year. Retest regularly and treat the dose as a living number.

Dosing methods compared

Two-part (balanced calcium and alkalinity)

Two-part systems are a pair of solutions, one with calcium (usually calcium chloride) and one with alkalinity (sodium bicarbonate or carbonate), formulated so equal volumes supply calcium and alkalinity in the right ratio. Randy Holmes-Farley's "Recipe 1", sold commercially by Bulk Reef Supply and others, is the best-known DIY version. Per its published strengths, about 1 mL per US gallon raises alkalinity by roughly 1.4 dKH, and the matching calcium part raises calcium by roughly 10 ppm. Most recipes also include a separate magnesium part.

Two-part is cheap, scalable and works with any tank size. Its one long-term drawback is that it slowly accumulates sodium chloride. Regular water changes take care of that.

Kalkwasser (limewater)

Kalkwasser is calcium hydroxide dissolved in RO/DI water. A saturated solution holds only about 1.5–1.7 g of calcium hydroxide per liter at room temperature, which limits how much it can supply, and its pH is about 12.4 when saturated. It is usually dripped slowly or dosed through the ATO in place of top-off water. Benefits include perfectly balanced calcium and alkalinity, a pH boost, and some precipitation of phosphate. The risk is overdose: a stuck ATO full of kalkwasser can spike pH to dangerous levels, so use a reservoir or doser sized so that a failure cannot empty too much at once.

Calcium reactors

A calcium reactor passes tank water and CO₂ through a chamber of aragonite media. The acidic water dissolves the media and releases calcium and alkalinity in the same ratio found in coral skeletons, plus magnesium and trace elements. Reactors suit large SPS-dominated tanks with high demand. They cost more up front, need a CO₂ tank and regulator, and take some patience to tune. Effluent pH and drip rate determine output.

All-in-one and "complete" solutions

Several manufacturers sell single-bottle or multi-bottle solutions that include trace elements. These are convenient for nano and medium tanks with modest demand, but read the label: concentrations vary widely between brands, so follow the manufacturer's dosing guidance, not a generic rule.

Correcting a low reading

When a parameter is low, raise it gradually. The commonly cited ceilings are about 1 dKH of alkalinity, 50 ppm of calcium and 100 ppm of magnesium per day. Dissolve dry chemicals in RO/DI first, add into high flow, and retest before dosing more. The calculator applies these limits automatically and tells you how many days to spread a large correction over.

For reference, the chemistry behind the calculator's numbers:

Reef-grade sodium bicarbonate (NaHCO₃, e.g. the alkalinity part of a 2-part kit): about 30 mg per liter raises alkalinity 1 dKH.
Reef-grade soda ash (sodium carbonate, Na₂CO₃): about 18.9 mg per liter per 1 dKH. Soda ash raises pH more sharply, so dose it slowly.
Calcium chloride dihydrate: about 3.67 mg per liter per 1 ppm calcium.
Magnesium chloride hexahydrate: about 8.37 mg per liter per 1 ppm magnesium. Epsom salt (magnesium sulfate heptahydrate) is about 10.14 mg per liter per 1 ppm, and is usually blended with magnesium chloride rather than used alone so sulfate doesn't build up.

Testing tips

Test alkalinity at the same time of day. Alkalinity can dip a bit overnight and rebound with lights on in some tanks.
Replace reagents by their expiry date. Old calcium and magnesium reagents are a common source of strange readings.
Cross-check occasionally with a second brand or an ICP test.
Log every result. Trends are far more useful than a single test. Coral Compass is building a free logger for exactly this.

Summary

Keep alkalinity steady in the 7–9 dKH range, calcium around 400–450 ppm and magnesium around 1,250–1,400 ppm. Measure your tank's daily consumption, replace it with a balanced method, and change your dose in small steps. When calcium and alkalinity will not cooperate, check magnesium.

Frequently asked questions

What should alkalinity be in a reef tank?

Most reefers target 7–9 dKH, with 8–8.5 dKH a very common choice. Values up to about 11 dKH can work if they are stable, but swings matter more than the exact number.

How much calcium is used per dKH of alkalinity?

Calcium carbonate formation uses about 20 ppm of calcium per 1 meq/L of alkalinity, which is roughly 7.1 ppm of calcium per 1 dKH.

Why won't my calcium and alkalinity stay up?

The most common hidden cause is low magnesium, which lets calcium carbonate precipitate onto equipment. Test magnesium and bring it to 1,250–1,400 ppm.

How fast can I raise alkalinity?

Common guidance is no more than about 1 dKH per day, split into several doses, with a retest before dosing more.

Should I dose calcium and alkalinity at the same time?

No. Add them hours apart into high-flow areas so local supersaturation does not cause precipitation.

Open the free dosing calculator

More reef science guides

Ranges are common hobby targets compiled from reef chemistry literature (including Randy Holmes-Farley's articles), hobby consensus and manufacturer documentation. They're not rules. Last reviewed 2026-10-15.