| 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.