Calcium & Magnesium: The Reef Chemistry Pair, and Why They’re Tested With KH
Calcium and magnesium are the other two legs of reef chemistry alongside KH, together often called “the big three.” Calcium is the main building block corals use for their skeletons; magnesium doesn’t get consumed nearly as fast, but it keeps calcium and carbonate (KH) from precipitating out of solution together, which is why letting magnesium drop causes the other two to become unstable and hard to dose accurately.
What Calcium and Magnesium Do in a Reef Tank
Calcium is drawn straight out of the water by stony corals, clams, and coralline algae as they build their calcium carbonate skeletons, so an established reef tank uses it up continuously and needs it replaced through dosing or water changes. Magnesium works differently: corals use very little of it directly, but it acts as a buffer in the water chemistry itself, keeping calcium and carbonate ions from bonding and dropping out of solution as a precipitate before corals ever get to use them. A tank with healthy calcium and KH numbers but low magnesium will often show cloudy dosing reactions or numbers that refuse to hold steady, because the water simply can’t carry that much dissolved calcium and carbonate without enough magnesium to keep them apart.
Target Levels and Why They’re Tested Together
Reef keepers typically target roughly 400 to 450 ppm calcium and 1250 to 1350 ppm magnesium, alongside 8 to 12 dKH. See KH (Alkalinity) for how carbonate hardness fits into the same picture. All three are chemically linked, so a change in one affects how stable the other two can be, and testing them as a single weekly routine catches an imbalance long before it shows up as a coral health problem. Natural seawater sits close to these same ranges, which is why they’re the benchmark most test kits and dosing charts are built around.
Common Mistake: Chasing One Number While Ignoring the Others
A frequent troubleshooting mistake is chasing a low calcium or KH reading with heavier dosing while magnesium is quietly low in the background. Since magnesium is what keeps calcium and carbonate from precipitating together, dosing more calcium or alkalinity into water that’s already short on magnesium limits how much of either the water can actually hold in solution, so the readings won’t hold no matter how much is added. The fix is to test magnesium alongside calcium and KH rather than in isolation, and to correct a low magnesium reading first if all three are off at once.
Correcting a Low Reading
A low calcium reading is usually corrected with a two-part dosing system, kalkwasser, or a calcium reactor, depending on tank size and how much livestock is drawing calcium down. A low magnesium reading is corrected with a dedicated magnesium supplement rather than more calcium or alkalinity product, since dosing the wrong one won’t move the number that’s actually short. Whichever method is used, dose gradually and re-test rather than dosing to a target number in one go. All three parameters interact, so a large single dose of any one of them risks pushing another out of range, and a slow correction over several days is easier to control than trying to fix a deficit overnight.
FAQ
What is a good calcium level for a reef tank?
Most reef keepers target 400 to 450 ppm, matching natural seawater. SPS-dominant tanks are usually kept toward the top of that range because of how much skeleton those corals lay down.
What is a good magnesium level for a reef tank?
Most reef keepers target 1250 to 1350 ppm. Magnesium is consumed much more slowly than calcium, but it still needs checking regularly since a low reading destabilizes both calcium and KH.
Why does magnesium matter if corals barely use it?
Magnesium keeps calcium and carbonate (KH) dissolved in the water instead of precipitating out together. Low magnesium is one of the most common, and most overlooked, reasons calcium and KH readings become hard to hold steady.
Can I dose calcium without checking magnesium first?
It’s better not to. If magnesium is low, added calcium and carbonate can’t stay in solution as effectively, so the calcium reading may not rise the way the dose suggests it should, and the KH reading can become unstable at the same time.
How often should I test calcium and magnesium?
Weekly is a reasonable starting point for most established reef tanks, tested alongside KH from the same water sample. Lightly stocked tanks can stretch the interval; SPS-heavy systems with high demand often test more frequently.
Shop Calcium & Magnesium Test Kits
The Salifert Calcium and Magnesium test kits cover both parameters, and the wider Marine Aquarium Water Conditioner range has dosing supplements for both. For the testing technique itself, see how to run the Salifert calcium test accurately, the same titration method the magnesium kit uses.