Reef Water Chemistry Explained: Alkalinity, Calcium and Magnesium

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Tanky

Most reef problems that look like a coral problem are really a chemistry problem. Learn three numbers and how they move together, and you can stop guessing why a coral is pale, slow or losing tissue.

Quick answer: Reef corals that build a stony skeleton need steady alkalinity (about 8–11 dKH), calcium (about 400–450 ppm) and magnesium (about 1,250–1,350 ppm) at a salinity of roughly 1.025 and a temperature of 24–26°C. Alkalinity is the one that matters most and moves fastest, so test it first and most often. A steady number beats a perfect number: avoid swings of more than about 1 dKH in a day, change one thing at a time, and retest before you dose again.

Why Chemistry Matters on a Reef
The Big Three: Alkalinity, Calcium, Magnesium
The Supporting Numbers
Target Ranges at a Glance
How and When to Test
Replacing What Corals Use Up
Dosing Safely
Troubleshooting by Symptom
Cairo Summers and Chemistry
FAQ

Reef Water Chemistry Explained: Alkalinity, Calcium and Magnesium

Why Chemistry Matters on a Reef

Stony corals, which include LPS and SPS types, build their skeletons from calcium carbonate. To do that they pull calcium and carbonate alkalinity out of the water, and they need magnesium to keep those two dissolved instead of crashing out as crust. Soft corals such as zoanthids do not build a heavy skeleton, so they use far less, which is one reason they are easier for beginners.

The key idea is that a reef tank is a closed box. Whatever the corals consume does not come back on its own, and that is why numbers drift downward over days. Your job is to measure the drift and replace what was used, gently and steadily. If you have not set up your tank yet, start with our reef setup guide.

The Big Three: Alkalinity, Calcium, Magnesium

Alkalinity (KH) – The Number That Matters Most

Alkalinity measures the water’s carbonate “buffer.” It is the raw material for coral skeletons and it also keeps pH from swinging. A target of about 8–11 dKH suits most reefs, and many beginners do well holding steady near the lower-middle of that range. Alkalinity is consumed fastest and changes fastest, so a stable alkalinity matters more than hitting a high one.

You will see three units. As a conversion, 1 meq/L equals about 2.8 dKH, and 1 dKH equals about 17.9 ppm. Check which unit your test kit reports so you compare like with like.

Warning: Rapid alkalinity swings stress and can kill stony corals. As a common rule of thumb, avoid changing alkalinity by more than about 1 dKH in a day, up or down.

Calcium – The Building Block

Natural seawater holds roughly 410–420 ppm of calcium at a salinity of 35 ppt, and a reef target of about 400–450 ppm suits most tanks. Calcium usually moves more slowly than alkalinity, because the tank holds a lot of it compared with what corals take each day.

Magnesium – The Quiet Referee

Magnesium keeps calcium and alkalinity from precipitating out of the water as cloudy crust. A target of about 1,250–1,350 ppm is typical. If magnesium runs low, you will find calcium and alkalinity hard to hold in range, no matter how much you dose. Test it every so often and correct it before chasing the other two.

How the Three Move Together

When a stony coral grows, it consumes calcium and alkalinity together, in a fixed ratio. In practice, dropping alkalinity by 1 dKH goes with using up roughly 7 ppm of calcium. Because seawater holds so much calcium and relatively little alkalinity, alkalinity is the one that runs out first. That is why most reef keepers watch alkalinity most closely and calcium less often.

Which of your corals use the most? Soft corals such as the Zoanthid Coral draw down very little. Stony LPS corals such as the Acan Brain Coral and Lobophyllia Coral use moderate amounts, and fast-growing SPS use the most. Add stony corals one at a time so you can see the effect on your numbers.

The Supporting Numbers

Salinity: keep it near 1.025 specific gravity (about 35 ppt), measured with a refractometer. Calcium, magnesium and alkalinity all scale with salinity, so a drifting salinity makes every other reading drift too.
Temperature: 24–26°C, steady. Warm water holds less oxygen and stresses corals long before it kills them.
pH: roughly 8.0–8.4, with a small daily rise and fall. A pH that sits very low usually points to low alkalinity or poor gas exchange.
Nitrate: low but not zero, roughly 2–10 ppm. High nitrate feeds algae; zero can leave corals pale.
Phosphate: low but not zero, roughly 0.01–0.08 ppm. Too much fuels algae that can smother coral.

The simple “ammonia, nitrite and nitrate” tests in our Water Parameters 101 guide still apply to a reef. The reef-specific numbers are the extra layer on top.

Target Ranges at a Glance

Hobby sources differ slightly, so treat these as ranges, not exact lines. What matters is that the readings stay steady from day to day.

Parameter	Target	What it does	If it drifts
Alkalinity	8–11 dKH	Skeleton material, pH buffer	Slow growth, tissue loss, pH swings
Calcium	400–450 ppm	Skeleton building block	Stunted stony coral growth
Magnesium	1,250–1,350 ppm	Keeps Ca and alkalinity dissolved	Cloudy water, hard to hold Ca and alk
Salinity	1.025 (35 ppt)	Base for every other value	Osmotic stress, all readings shift
Nitrate	About 2–10 ppm	Nutrient for algae and zooxanthellae	High: algae. Zero: pale corals
Phosphate	About 0.01–0.08 ppm	Nutrient, also affects skeleton growth	High: algae. Very low: stalled growth

How and When to Test

New tank, soft corals only: test alkalinity, calcium, magnesium, salinity and nitrate weekly.
After adding a stony coral: test alkalinity daily for the first week so you see how fast the tank uses it.
Established stony-coral tank: test alkalinity every few days, and calcium and magnesium weekly.
Test at the same time of day, because alkalinity and pH shift slightly through the lighting cycle.
Write every result down. The trend tells you more than any single reading, and a log makes a slow downward drift obvious before corals show it.
Use fresh reagents and follow the instructions exactly, including the sample volume and timing. Old or sloppy tests are the most common reason for “mystery” readings.
Double-check surprising results by testing again, or with a second kit, before you react.

Tip: If you do not want to run tests yourself yet, our Marine & Reef Aquarium Maintenance visits include the test reagents. Marine Pro adds KH and phosphate checks, and Reef Prime tests KH, phosphate, calcium and magnesium and keeps a log.

Replacing What Corals Use Up

There are three levels of reef chemistry management, and most keepers move up as their coral collection grows.

Water changes only: a regular change with fresh, properly mixed saltwater replenishes calcium, magnesium and alkalinity together. For a tank with mostly soft corals or a few small LPS, this is often enough. Our water change guide covers the routine.
Supplements: when stony corals use more than water changes replace, you add calcium and alkalinity separately, using a “two-part” solution or an all-in-one supplement, following the product’s directions.
Automation: larger or SPS-heavy tanks often move to automatic dosing pumps, a calcium reactor, or kalkwasser (limewater) added through the top-off water, so supply tracks consumption without daily manual work.

Whatever you choose, the supplement should match measured consumption, not a calendar. Dosing a fixed amount “just in case” is how numbers climb out of range.

Dosing Safely

Test first, then dose. Never add a supplement without a fresh reading.
Change one parameter at a time. Adjust alkalinity or calcium, retest, then move on.
Small steps: raise alkalinity by well under 1 dKH per day, and spread larger corrections over several days.
Add solutions slowly, into a high-flow area such as near a pump outlet, so they mix at once rather than forming a cloud.
Keep calcium and alkalinity doses apart in time and place. Dosed together in a still spot they can precipitate, leaving cloudy water and lower readings than you expected.
Fix magnesium first if it is low, because low magnesium makes the other two hard to hold.
Follow the product label, and remember that “more” is never the answer in a closed system.

Flow matters here too. Strong, even circulation mixes anything you add and carries nutrients to every coral, which is why a wave maker is a core piece of reef equipment.

Equipment that supports stable chemistry: the Jebao RW-4 Wave Maker Pump mixes dosed solutions quickly and keeps flow even, and the Bubble Magus Curve 5 Protein Skimmer removes dissolved waste before it turns into nitrate and phosphate.

Troubleshooting by Symptom

What you see	Likely chemistry cause	First step
Stony coral losing tissue	Alkalinity swing or a drop	Test alkalinity now and stop any large corrections
Slow or stalled growth	Low alkalinity, calcium or magnesium	Test all three and fix magnesium first if low
Cloudy white water after dosing	Precipitation from dosing too fast or too close together	Pause dosing, increase flow, retest in a day
Pale or bleached coral	Heat, too much light, or nutrients that are too low	Check temperature, then nitrate and phosphate
Algae outbreak	High nitrate or phosphate	Test both, water change, check skimmer and feeding
Every reading keeps drifting	Salinity is moving, often from evaporation	Check salinity and top off with fresh RO/DI water

Cairo Summers and Chemistry

Hot weather affects chemistry in two ways. First, evaporation speeds up, which pushes salinity and every dissolved value upward if you do not top off with fresh RO/DI water. Second, warmer water holds less oxygen and puts corals under stress, so a small chemistry slip hurts more. Keep the tank cool, top off daily, and read How to Cool Your Aquarium in Summer Heat before the first heatwave. Evaporative fans work, but they raise evaporation further, so check salinity more often when you use them.

Summer cooling: the Aquarium Cooling Fan Set comes with 1 to 4 fans and an optional temperature sensor for automatic control. If you want ready-made water for a change, the maintenance plan page notes that RO water, marine salt or ready-mixed saltwater can be bought from us at a separate cost.

Frequently Asked Questions

What is the most important reef parameter?
Alkalinity. It is consumed fastest, it drives pH stability, and sudden swings are the most common chemistry cause of stony coral problems.

What should my alkalinity be?
Roughly 8–11 dKH for most reefs. Choose a value in that range and hold it steady rather than chasing the top of it.

How often should I test reef water?
Weekly for a soft-coral tank, every few days for alkalinity in a stony-coral tank, and daily for the first week after adding a new stony coral.

Do soft corals need calcium and alkalinity dosing?
Usually very little. A tank with mostly soft corals is often maintained with regular water changes and weekly testing.

Why does my water go cloudy when I dose?
Calcium and alkalinity supplements can precipitate if added too fast or in the same spot. Slow down, add them in a high-flow area, and separate the doses.

What is the difference between dKH, meq/L and ppm?
They are different units for alkalinity. As a conversion, 1 meq/L equals about 2.8 dKH, and 1 dKH equals about 17.9 ppm.

Can I fix chemistry quickly with a big water change?
A large change can shock corals if the new water differs from the tank. Make corrections gradually, and match salinity and temperature when you change water.

Conclusion

Three numbers, one habit. Keep alkalinity, calcium and magnesium in range and steady, test on a schedule, log the results, and dose only what you have measured. A reef that never swings is a reef where corals grow.

Reef Water Chemistry Explained: Alkalinity, Calcium and Magnesium

Table of Contents

Reef Water Chemistry Explained: Alkalinity, Calcium and Magnesium

Why Chemistry Matters on a Reef

The Big Three: Alkalinity, Calcium, Magnesium