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Why dose magnesium before calcium and alkalinity
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.
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.
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.
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.
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.