Saltwater & Reef
An explainer on the big three reef chemistry parameters, how they interact, and how to keep them stable for healthy coral growth.
When people ask me why their corals look sulky despite a spotless-looking tank, my first question is almost never about lighting or flow. It's about the "big three" — alkalinity, calcium, and magnesium — because these three parameters do more day-to-day work in a reef than almost anything else you can measure. Understanding how they relate isn't complicated once you see the underlying chemistry, and getting comfortable with them is the single biggest step most new reefers can take toward corals that actually grow instead of just surviving.
Stony corals, clams, coralline algae, and a surprising number of other reef organisms build their structures out of calcium carbonate. To do that, they pull two ingredients out of your water: calcium ions and carbonate (which we measure indirectly as alkalinity). Magnesium is the quiet third partner — it doesn't get built into skeletons in large amounts, but it controls whether calcium and carbonate stay dissolved and available in the first place.
Here's the mental model I use: calcium and alkalinity are the bricks and mortar, and magnesium is the scaffolding that keeps the whole worksite from collapsing prematurely. If you have plenty of bricks but the mortar keeps crashing, nothing gets built. If the scaffolding is missing, the bricks and mortar react with each other and fall to the floor before the corals can use them.
That's the real reason these three are discussed together instead of in isolation. They're chemically linked, and a problem with one almost always shows up as a problem with another.
If you only test one thing regularly, test alkalinity. It's the most dynamic of the three and usually the first to signal trouble.
Alkalinity is a measure of your water's buffering capacity — roughly, its ability to resist swings in pH. In a reef we mostly care about it because carbonate and bicarbonate are the carbon source corals use for their skeletons. As corals grow, they consume alkalinity, and in a well-stocked SPS tank that consumption can be surprisingly quick.
Alkalinity gets reported two ways, which trips up a lot of newcomers:
The conversion is simple: 1 meq/L equals roughly 2.8 dKH. Natural seawater sits around 7 to 8 dKH. Most reefers run somewhere in the 8 to 9 dKH range, and many mixed and SPS-dominant tanks are perfectly happy there. You can run higher, but I'll get to why I don't chase that.
The number I actually care about isn't a single reading — it's the rate of consumption. Test on Monday, test again a couple of days later without dosing, and see how far alkalinity has dropped. That daily consumption figure tells you how much you need to dose to hold steady, and it's the foundation for any automated dosing later on. A tank drawing down 0.5 dKH per day needs very different handling from one drawing down 2.0.
Calcium tends to worry beginners the most because the numbers are large — we're talking hundreds of parts per million — but it's honestly the most forgiving of the three on a day-to-day basis.
A typical target is 400 to 450 ppm, with natural seawater around 420. The reason calcium feels stable is simply that there's so much of it dissolved relative to how fast corals consume it. A reef holds a big reservoir of calcium, so even active growth moves the number slowly. I've seen tanks coast for a couple of weeks with only minor calcium drift while alkalinity needed daily attention.
That said, calcium and alkalinity are consumed in a roughly fixed ratio as corals lay down calcium carbonate. So if your alkalinity is being pulled down steadily, your calcium is being consumed too, even if the large reservoir masks it. This is why I don't treat calcium as an afterthought — I test it every week or two and correct it before it drifts far.
One practical caveat: chasing a high calcium number does nothing useful. Above roughly 450 ppm you gain no benefit and you increase the risk of unwanted precipitation, especially if alkalinity is also high. More is not better here.
Magnesium is the parameter people ignore until something inexplicable goes wrong, and then it turns out to be the culprit. It's present in seawater at around 1250 to 1350 ppm, far higher than calcium, and it doesn't get consumed nearly as fast — so it can sit untested for a long time and quietly drift low.
Calcium and carbonate really "want" to combine and precipitate out of solution as solid calcium carbonate. Magnesium interferes with that process. It essentially poisons the growing crystal surfaces of abiotic (non-biological) calcium carbonate, keeping calcium and alkalinity dissolved and available for your corals instead of caking onto heaters and pump impellers.
When magnesium drops too low, you see the classic symptom: you dose calcium and alkalinity, but neither number will hold. They keep precipitating out. Reefers burn through test kits and reagents trying to force the big two into range, when the actual fix is topping up magnesium first.
The link between these parameters is where the real understanding lives. A few relationships I keep in mind:
That third point is the trap I see most often with eager reefers. The instinct is to run everything at the high end "for maximum growth," but the higher you push calcium and alkalinity simultaneously, the smaller your margin for error becomes.
This is the part I wish someone had drilled into me earlier. A stable 7.8 dKH beats a 9.0 that swings to 7.5 and back every week. Corals — especially SPS — adapt to the conditions they've been living in. They tolerate a level far better than they tolerate change.
The most common cause of tissue recession, faded color, and "burnt" SPS tips in otherwise healthy-looking tanks isn't a wrong target number. It's an alkalinity swing. A big water change with a mismatched salt, a dosing pump that ran dry, or a burst of coral growth that outpaced your dosing can all move alkalinity fast enough to do visible damage.
So my genuine advice is: pick a target you can realistically hold, and then defend it.
You don't need a laboratory. Here's the cadence that has served me and most reefers I know well:
A couple of honest caveats about testing itself. Hobby test kits have real margins of error, and different brands can disagree by a full point or more — especially on alkalinity. The trick is to stay with one kit and watch the trend, rather than swapping brands and chasing the difference between them. Test at the same time of day when you can, since pH and alkalinity shift a little over the daily cycle. And replace reagents when they age; an old, oxidized bottle will lie to you convincingly.
The big three aren't complicated once you stop treating them as three separate chores and start seeing them as one connected system. Calcium and alkalinity are the raw materials your corals consume as they grow, magnesium is what keeps those materials usable, and stability is the thread that ties it all into healthy growth.
If you're just starting out, keep it simple: get magnesium into range and leave it, hold calcium in the 400s, and then focus your real attention on measuring and defending a steady alkalinity. Learn your tank's daily consumption, dose to match it, and resist the urge to chase impressive numbers. Corals reward consistency far more than they reward ambition — and once you internalize that, the big three stop being intimidating and start being routine.
Mei runs a mixed reef and has the test logs to prove how much she respects stability. She explains saltwater keeping honestly — the costs, the patience and the payoff — so newcomers go in with clear eyes and healthy corals.
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