Most reef tanks aim for calcium between 380 and 450 mg/L; natural seawater is about 420 mg/L. Corals use it together with alkalinity, so the two usually fall together. If it’s low, raise it with calcium chloride, by no more than about 50 mg/L a day.
Why calcium matters
Stony corals, clams and coralline algae build their skeletons from calcium carbonate. They take calcium and alkalinity from the water at the same time, in a fixed ratio: for every 1 dKH of alkalinity used, about 7 mg/L of calcium goes with it.
That ratio is useful. If alkalinity drops 1 dKH a week, calcium should drop about 7 mg/L a week. If calcium is falling much faster or slower than that, check your test or look for another cause.
Too high, too low
Holmes-Farley suggests 380–450 mg/L. Below about 380, coral growth can slow. Well above 450, especially with high alkalinity, calcium carbonate can come out of solution as a crust on heaters and pumps, taking both with it.
Calcium moves more slowly than alkalinity, so a sudden change in your reading is more often a testing issue than a real swing. Retest before acting.
How much calcium chloride to add
Most hobby calcium additives are calcium chloride dihydrate. By its chemistry, about 3.7 g raises 100 L by 10 mg/L. Bulk Reef Supply suggests raising calcium by no more than 50 mg/L a day, dissolving it in RODI water first and dosing slowly into a high-flow area.
| Raise by | 100 L | 200 L | 300 L |
|---|---|---|---|
| 10 mg/L | 3.7 g | 7.3 g | 11 g |
| 25 mg/L | 9.2 g | 18.3 g | 27.5 g |
| 50 mg/L | 18.3 g | 36.7 g | 55 g |
Calculated from the molecular weights. Anhydrous calcium chloride is stronger, so check which you have; for a branded product, use its label dose.
Keep it steady
Once calcium is in range, it usually stays there if you replace alkalinity in balance: a two-part system, a calcium reactor or kalkwasser all add both together. Our guides to two-part dosing and calcium reactors vs dosing pumps compare the options.
Test calcium every one to two weeks alongside alkalinity, and keep magnesium in range too, because low magnesium makes calcium and alkalinity harder to hold. See the magnesium guide.
Common questions
Is 500 calcium too high?
It’s above the usual range. If alkalinity is also high, precipitation is likely. Stop adding calcium and let it fall slowly.
Why is my calcium low but alkalinity fine?
Check your test first. Then check magnesium, and whether your dosing adds calcium and alkalinity in balance.
Do water changes keep calcium up?
In tanks with few stony corals, often yes. Once corals grow, dosing is usually needed.
Sources and limits
Last checked 12 October 2026. Ranges and doses here come from the published sources below and from the chemistry of each additive. They’re starting points for your own tank, not guarantees for any coral.
- Randy Holmes-Farley, Reefkeeping: Reef Aquarium Water Parameters
- Bulk Reef Supply: calcium chloride dosing instructions
Keep going
Work out your daily use · Free Reef Companion · All calculators