| Why They Matter | Corals build skeletons from them |
| Alkalinity | 8–9 dKH (aim for stability) |
| Calcium | 400–450 ppm |
| Magnesium | 1250–1350 ppm |
| Golden Rule | Stability beats perfect numbers |
Overview
Alkalinity, calcium, and magnesium are the three parameters that matter most once you keep corals. Corals pull calcium and carbonate (measured as alkalinity) out of the water to build their skeletons, and magnesium keeps both available in solution so they don’t precipitate out. In a tank with growing corals these are consumed daily and must be replaced — this is what “dosing” a reef tank is really about.
What Each One Does
Alkalinity (dKH) is your carbonate buffer — it feeds skeleton growth and stabilizes pH. It’s the parameter corals are most sensitive to, and the one that swings fastest. Calcium is the other half of the skeleton (calcium carbonate). Magnesium is the referee: at proper levels it keeps calcium and alkalinity from binding together and falling out of the water, which is why you can’t hold cal and alk up if mag is low.
Target Numbers
Aim for alkalinity 8–9 dKH, calcium 400–450 ppm, and magnesium roughly 3x your calcium (1250–1350 ppm). But the single most important rule in reef chemistry: stability beats the “perfect” number. A rock-steady 7.8 dKH grows better corals than an alkalinity that bounces between 8 and 11. Pick a target and hold it.
How to Keep Them Up
In a lightly-stocked tank, regular water changes with a quality salt mix replace what corals use. As coral demand grows you’ll add a supplement method — two-part dosing, kalkwasser, or a calcium reactor — sized to your tank’s daily consumption. Test alkalinity a couple of times a week (it moves fastest), and calcium/magnesium less often.
Common Mistakes
Chasing numbers with big corrections: a sudden alkalinity swing can burn coral tissue (“alk burn”) — adjust slowly, no more than about 1 dKH per day. Ignoring magnesium: when cal and alk mysteriously won’t stay up, low mag is usually why. Dosing blind: never add supplements without testing first — you’re dosing to consumption, not on a schedule someone else uses.
Related Guides
How to Dose Two-Part Solution · Acropora · Hammer Coral
Frequently Asked Questions
What alkalinity, calcium, and magnesium levels should I target in a reef tank?
For a mixed reef, aim for alkalinity around 8 to 9 dKH, calcium 400 to 450 ppm, and magnesium 1300 to 1350 ppm. Stability matters more than hitting the exact top of each range — steady numbers keep corals healthier than perfect readings that swing up and down.
Which parameter should I get in range first?
Magnesium. It keeps calcium and alkalinity dissolved in the water, so if magnesium is low you will fight to raise or hold the other two no matter how much you dose. Check and correct magnesium before chasing the other two.
Why does my alkalinity keep dropping?
Growing corals pull alkalinity and calcium out of the water to build their skeletons, so a healthy reef steadily consumes it — the faster your corals grow, the faster it falls. The fix is to replace what they use through regular dosing or a calcium reactor.
Can I dose calcium and alkalinity at the same time?
Dose them separately, in different spots and a few hours apart. Never combine concentrated calcium and alkalinity or pour them into the same place at once — they react and precipitate out as a cloudy snowstorm, cancelling each other out and wasting both.