Parameter Reference
CalciumCa
Calcium is the cation half of the CaCO₃ skeleton equation. The reef hobby fixates on this number, sometimes at the expense of alkalinity and magnesium. Below 380 ppm corals struggle; between 380–450 they grow well; above 500 you're courting precipitation. Most well-run reefs sit between 400–440 and rarely think about Ca specifically.
Target ranges
| Beginner-safe | 380–450 ppm |
| Optimal (consensus) | 400–450 ppm |
| Triton method target | 420–440 mg/L |
| Natural seawater | 400–412 ppm |
Why it matters
Coral, coralline algae, clams, and Halimeda all build calcium-carbonate skeletons. They consume Ca and alkalinity in roughly equimolar amounts (1:1 by ion count, ~1 ppm Ca per 0.14 dKH alk). This is why a tank that drops 1 dKH per day also drops about 5–6 ppm Ca per day. Dose them together or you create imbalance.
Ca is much slower-moving than Alk in terms of perception — alk swings 1 dKH look dramatic on a test kit but represent the same amount of total ion movement as a 7 ppm Ca change, which barely registers visually.
Symptoms of drift
Too low
Below 380 ppm: slow coral and coralline growth, weakened skeletons. Below 360 ppm corals, clams, and Halimeda all get progressively worse (Randy). Coralline algae stop spreading first — early visual indicator.
Too high
Above 500 ppm: precipitation risk, especially if alkalinity is also elevated. "Snowstorm" event if Ca and Alk doses overlap in time at high concentrations — the water turns cloudy white with CaCO₃ particulates.
Testing
Salifert Calcium and Red Sea Calcium Pro are the workhorse titration kits — 5 ppm resolution if you titrate slowly. Hanna HI758 Calcium Checker is faster but expensive per test. Test once a week alongside alkalinity. Drift is slow so weekly cadence is plenty unless you're commissioning new dosing.
FAQ
- What if Ca is at 380 but Alk is at 11?
- Don't dose Ca to chase up — raise Mg first if it's low, then bring Alk down to 9–10 over a few days, then let consumption pull both into balance. Force-dosing Ca at high Alk triggers precipitation.
- Why doesn't pure CaCl₂ dosing work long-term?
- Calcium chloride alone adds Ca²⁺ and 2 Cl⁻ for every dose. Long-term you build chloride imbalance. Pairing with NaHCO₃ (the alk side) gives you NaCl as the net residue — the salt your salt mix already contains. Water changes purge the small drift.
Sources & references
- 01Randy Holmes-Farley — Optimal Parameters for a Coral Reef Aquariumhttps://www.reef2reef.com/threads/optimal-parameters-for-a-coral-reef-aquarium-by-randy-holmes-farley.173563/
- 02Randy Holmes-Farley — A Homemade Two-Part Calcium and Alkalinity Additive Systemhttps://reefs.com/magazine/aquarium-chemistry-a-homemade-two-part-calcium-and-alkalinity-additive-system/
- 03Triton — CoreChemhttps://www.triton.de/en/products/corechem
- 04Global Seafood Alliance — Typical chemical characteristics of full-strength seawaterhttps://www.globalseafood.org/advocate/typical-chemical-characteristics-of-full-strength-seawater/
- 05Red Sea — Foundation Complete Manualhttps://redseafish.com/wp-content/uploads/2020/11/24653_Manual-Foundation-Complete-GB-_v21a-WEB.pdf
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