Reef Chemistry

Why dose magnesium before calcium and alkalinity

Updated 2026 3 min read

If your Ca and Alk readings won't hold no matter how much you dose, low magnesium is almost always the reason.

It's one of the most common frustrations on reef forums: you dose calcium and alkalinity, re-test the next day, and the numbers have barely moved — or they've fallen. Before you dose more (and risk a swing), check your magnesium. When Mg is low, calcium and alkalinity will not stay in range, and adding more of either just wastes supplement.

The chemistry, briefly

Calcium and carbonate (alkalinity) naturally want to combine and precipitate out of saltwater as calcium carbonate. Magnesium is what keeps them in solution — Mg ions interrupt that precipitation, letting Ca and Alk coexist at reef levels. When magnesium drops too low, Ca and Alk precipitate against each other faster than you can dose them, so both readings sag and refuse to climb.

The rule:

If magnesium is below range, bring it up first. Then re-test and dose calcium and alkalinity. Dosing Ca/Alk into a low-Mg tank is pouring supplement down the drain.

Target ranges

Targets vary a little by what you keep. As a reference (mixed reef as the default):

Parameter	Mixed reef	SPS-heavy	LPS / soft
Magnesium	1280–1400 ppm	1300–1400	1250–1400
Calcium	420–450 ppm	420–440	380–450
Alkalinity	8.0–9.0 dKH	7.5–8.5	8.0–11.0

Roughly, healthy natural-seawater-like ratios sit near Mg ~1300, Ca ~420, Alk ~8 dKH. SPS-dominant tanks run alkalinity a touch lower and tighter; LPS and soft-coral tanks tolerate a wider band.

Going slow matters more than going high

Once Mg is in range and you're dosing Ca and Alk, the danger isn't being slightly low — it's swinging. Alkalinity especially: keep daily increases gentle. A widely-used safe ceiling is no more than ~1.4 dKH of alkalinity rise per day; split larger corrections across several days. Sudden alkalinity swings stress corals and are a classic cause of tank crashes and tissue burn on SPS.

Never dose calcium and alkalinity at the same spot, at the same time.

Added together in a concentrated pocket, they precipitate immediately — clouding the water and wasting both. Dose them at different points in the water column (or different times) and let your flow distribute each one.

Gear this guide uses

You can't dose in the right order without confirming Mg, Ca, and Alk first — then correcting each.

Find low magnesium with a reef magnesium test kit, and check the other two with a calcium and alkalinity test kit.
Bring Mg up first with a reef magnesium supplement before you touch the other two.
Then hold Ca and Alk in range with a two-part calcium and alkalinity dosing set.

Some links below are affiliate links — if you buy through them, Moon Dog may earn a small commission at no extra cost to you. We only point to gear we'd actually use.

The app for this

Reef does the dosing math — in the right order

Enter your test readings and Reef recommends magnesium first when it's low, then paired calcium + alkalinity doses for your exact tank volume and supplement — with daily-rise caps built in so it never tells you to swing too hard. Works with Red Sea Foundation ABC+, Brightwell, ESV B-Ionic, Tropic Marin All-For-Reef, or a custom recipe. Pay once, no subscription, works offline.

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Sources

Randy Holmes-Farley — reef-aquarium water chemistry articles (Reef2Reef / Advanced Aquarist)

Bulk Reef Supply (BRS) — two-part dosing and magnesium guides

Reef2Reef community consensus on Mg / Ca / Alk balance

This is general reef-keeping guidance. Always confirm with your own test kits, dose gradually, and re-test after every correction.