Alkalinity, Calcium & Magnesium: Reef Dosing Explained

Reef tank alkalinity is the backbone of stable coral growth. Alongside calcium and magnesium, it supplies the carbonate hardness your stony corals use every day. When these three parameters drift, growth slows, tissue can recede and your water chemistry becomes hard to steer. This guide explains what each number means, typical reef targets in metric units, how the three parameters interact, and how to dose them safely in a New Zealand home reef.

What is reef tank alkalinity?

Alkalinity is a measure of how much acid your water can buffer before the pH drops. In reefkeeping we usually report it in degrees of carbonate hardness (dKH). Corals, coralline algae and many invertebrates pull carbonate and bicarbonate out of the water to build skeleton. As they grow, alkalinity falls.

If alkalinity swings up and down, coral tissue often looks stressed even when calcium looks fine on paper. That is why reef tank alkalinity is the parameter we watch most closely once a tank is stocked with stony coral. For the wider chemistry picture, see our water chemistry guide.

Calcium and magnesium: the other two pillars

Calcium is the other half of the coral skeleton equation. Stony corals combine calcium ions with carbonate to form calcium carbonate. Magnesium sits in the background: it helps keep calcium in solution and supports a stable alkalinity reading. When magnesium is low, calcium and alkalinity become harder to keep up together, and precipitation on heaters and pumps is more common.

Think of the three as a triangle. Change one side a long way and the other two will pull. That is why we rarely recommend dosing calcium hard while ignoring alkalinity, or chasing a perfect calcium number while alkalinity is crashing overnight.

Parameter	Typical mixed-reef target	Unit	Why it matters
Alkalinity	7–9 (choose one value and hold it)	dKH	Buffers pH and feeds coral skeleton growth
Calcium	400–450	mg/L (ppm)	Builds calcium carbonate skeletons
Magnesium	1,280–1,380	mg/L (ppm)	Keeps Ca and Alk stable in solution
Salinity	1.025–1.026 SG / ~35 ppt	SG or ppt	All ionic targets assume stable salinity
Temperature	25–26	°C	Stable temp keeps metabolism and chemistry predictable

These ranges suit most mixed reefs. Ultra-low nutrient SPS systems sometimes run alkalinity a little higher and tighter; soft-coral or fish-only systems may need less daily dosing. The important part is consistency, not chasing a textbook midpoint every afternoon.

How the three parameters interact

Corals consume alkalinity and calcium in a roughly fixed ratio. As a rule of thumb, when alkalinity drops by about 1 dKH, calcium often falls by roughly 7–8 mg/L if nothing else is going on. Magnesium is consumed more slowly, so it usually needs less frequent top-ups.

Practical implications:

• If alkalinity falls every day but calcium barely moves, your test kit, salt mix, or dosing ratio may be off. Recheck with a fresh kit before changing large doses.
• If both alkalinity and calcium fall together after you add more SPS, demand has gone up. Increase dosing gradually after testing for a few days.
• If magnesium is well below target, raise it first (slowly), then rebalance calcium and alkalinity. Low magnesium makes the other two fight you.

Always confirm salinity before chasing Ca/Alk/Mg. A tank mixed a little weak or strong will throw every reading off. Our guide on how to mix saltwater for a reef tank covers that step.

Testing reef tank alkalinity, calcium and magnesium

You cannot dose what you cannot measure. Titration kits (such as Red Sea’s Pro series) are still the everyday standard for home reefers. Digital checkers are useful once you trust your method. For a full testing routine, see our reef tank water testing guide.

• Alkalinity: test daily or every second day when you first start dosing; then 2–3 times a week once stable.
• Calcium: test 2–3 times a week at first, then weekly once the tank is steady.
• Magnesium: weekly is usually enough for a mature tank; more often while you are correcting a deficit.

Test at the same time of day. Alkalinity can drift a little between lights-on and lights-off. Write the results down so you can see the trend, not just today’s number.

When should you start dosing a reef tank?

New tanks often stay near target for weeks if you do regular water changes with a reef salt that is already balanced for calcium, alkalinity and magnesium. Dosing becomes useful when:

1. You are stocking more stony coral and see alkalinity falling between water changes.
2. Weekly water changes alone cannot keep your chosen alkalinity target steady.
3. You want smaller, daily top-ups instead of big weekend corrections.

If a brand-new tank is still cycling, focus on the nitrogen cycle first. Our guide on how to cycle a saltwater aquarium covers that stage. Heavy coral dosing before the tank is mature usually creates more problems than it solves.

Dosing reef tank alkalinity, calcium and magnesium

There are several sound approaches. The two we see most often in NZ reefs are Red Sea’s Reef Foundation system and Fauna Marin’s Balling Light method. Both replace what corals consume; they differ in how the solutions are mixed and which trace elements ride along.

Red Sea Reef Foundation (A, B and C)

Red Sea splits the big three into three bottles:

• Foundation A — calcium
• Foundation B — alkalinity (carbonate hardness)
• Foundation C — magnesium

You test, then dose each part according to the label for your tank volume and the deficit you measured. Many reefers start with manual daily doses, then move the same solutions onto a dosing pump once the daily amounts are predictable. Red Sea also publishes programme guidance tied to their salt and coral-colours range; use the instructions that match the products you actually run. For a deeper look at their newer multi-part approach, see Red Sea’s 4-part reef supplement system.

Fauna Marin Balling Light

Balling Light uses separate calcium, carbonate and magnesium salt mixes (plus optional trace solutions). You dissolve the powders into RODI water, then dose the solutions to match consumption. It is flexible and popular with keepers who want fine control over each parameter. Follow Fauna Marin’s mixing and dosing instructions for each mix; do not invent your own ratios.

A safe dosing workflow

1. Test alkalinity, calcium and magnesium (and confirm salinity).
2. Choose one alkalinity target inside the 7–9 dKH band and write it on your log.
3. Correct large deficits slowly. Raise alkalinity in small steps across a day or two rather than one big pour. Follow the maximum adjustment guidance on your product label.
4. Dose into a high-flow area of the sump or display, never onto coral tissue, and keep calcium and alkalinity solutions from mixing in the same tubing or container.
5. Split daily demand into several smaller doses if you use a pump. Smooth dosing reduces pH and precipitation spikes.
6. Retest the next day at the same time. Adjust the daily amount based on the trend, not a single surprising reading.

Water changes still matter

Even with solid dosing, regular water changes reset minor ions, organics and the small imbalances that build up over weeks. A quality reef salt already carries balanced calcium, alkalinity and magnesium, so each change is a soft reset. Browse our Red Sea salt range and the wider marine salt collection when you restock. Pair that with the supplements in our Red Sea Reef Care and Fauna Marin Balling Light collections.

Common dosing mistakes

• Dosing without testing. Guessing daily mL is how tanks overshoot alkalinity and crash pH overnight.
• Big catch-up doses. Corals prefer a flat line. Spread corrections out.
• Mixing calcium and alkalinity in one container. They can precipitate into useless white sludge. Keep solutions separate.
• Ignoring magnesium. A low Mg reading makes Ca and Alk look “unstable” for weeks.
• Changing salt mix and dosing recipe at the same time. Change one variable, then retest for a week.
• Out-of-date reagents. Old titration kits lie. Replace reagents on the maker’s schedule.

Frequently asked questions

What alkalinity should my reef tank be?

Most mixed reefs do well between 7 and 9 dKH. Pick one number in that band and keep it steady. Stability matters more than hitting a particular decimal.

How often should I test reef tank alkalinity?

While you are learning your tank’s demand, test daily or every second day. Once dosing matches consumption, two or three checks a week is usually enough.

Can I dose calcium and alkalinity at the same time?

You can dose both on the same day, but add them in different places or at different times so the concentrates do not meet in a small volume of water. Never premix them in one jug.

Do soft-coral tanks need the same dosing?

Soft corals and a light fish load often get enough from water changes alone. Start dosing when your tests show alkalinity or calcium falling between changes.

Is Balling Light the same as Red Sea Foundation?

Both replace calcium, alkalinity and magnesium. Balling Light is a salt-mix method with optional traces; Reef Foundation is a ready liquid three-part system. Choose one approach and follow that brand’s instructions rather than mixing recipes.

Unsure what your numbers are telling you? Bring a water sample into our Auckland store at Unit 21, 114 Wiri Station Road, Wiri, or contact our team. We are happy to talk through a testing and dosing plan that fits your tank.

Last reviewed: 6 November 2026.