Alkalinity, Calcium & Magnesium Explained
The reef chemistry trio demystified — what alkalinity, calcium and magnesium do, ideal ranges, how they interact, and how to dose them safely.
If reef chemistry feels intimidating, start here. Three parameters do most of the heavy lifting for coral growth — alkalinity, calcium and magnesium — and once you understand how they relate, dosing stops feeling like alchemy.
What each one does
Alkalinity (dKH) is the water’s buffering capacity — its ability to resist pH swings. Corals consume carbonate from alkalinity to build their skeletons, so it’s the parameter that drops fastest in a growing reef. Target 8–9 dKH.
Calcium (ppm) is the other half of the coral skeleton (calcium carbonate). Target 400–450 ppm.
Magnesium (ppm) is the quiet enabler. It keeps calcium and alkalinity from precipitating out of the water. Without enough magnesium, you’ll fight to keep the other two stable. Target 1300–1400 ppm.
How they interact
Here’s the key insight beginners miss: calcium and alkalinity are linked. As coral pulls them from the water to grow, both drop together. Magnesium acts as the referee that keeps them dissolved and available.
If your alkalinity and calcium won’t hold steady no matter how you dose, check magnesium first. Low magnesium is the usual culprit.
Test before you dose
Never dose blind. Use quality test kits and measure before adding anything:
- Test alkalinity, calcium and magnesium.
- Compare to target ranges.
- Dose only what’s needed to close the gap — slowly.
Alkalinity is the one to watch most closely. Don’t raise it more than ~1 dKH per day — fast swings stress and can burn coral tissue.
Establishing a dosing routine
As your reef grows, it will consume these elements at a predictable daily rate. The workflow:
- Measure the daily drop in alkalinity over a few days.
- Dose that amount daily to hold it steady (a dosing pump automates this).
- Re-test weekly and adjust as coral mass increases.
Two-part dosing (separate alkalinity and calcium solutions) is the most common beginner-friendly method. Larger systems often graduate to a calcium reactor.
A simple troubleshooting table
| Symptom | Likely cause | First check |
|---|---|---|
| Alk & Ca won’t hold | Low magnesium | Test & raise Mg |
| Alk drops fast | Heavy coral growth | Increase dosing |
| Cloudy water after dosing | Dosing too fast / too much | Slow down, smaller doses |
| pH low | Low alkalinity / CO₂ | Test alk, improve aeration |
Master these three and you’ve mastered the foundation of coral health. For the bigger picture, revisit our coral care fundamentals, and when your chemistry is dialed in, add some new coral to put it to work.