What calcium does in a reef tank

Calcium ions (Ca2+) combine with carbonate ions (from alkalinity) to form calcium carbonate — the building block of every stony coral skeleton, every coralline algae crust, and every snail or clam shell. Without adequate calcium, stony corals stop growing and existing skeletons can dissolve over time.

Calcium is consumed continuously as corals build skeleton. The faster a tank grows, the more calcium it consumes. Soft corals and mushroom-dominant tanks consume little. Mixed reefs consume moderately. SPS-dominant tanks consume the most.

Target calcium range

Standard target is 380 to 450 ppm calcium. Most reef keepers aim for 410 to 440 ppm as the sweet spot. Above 480 ppm starts to interfere with alkalinity stability and can cause abiotic precipitation (cloudy water, white film on pumps).

As with alkalinity, stability matters more than the exact number. A steady 415 ppm Ca for months will out-grow a tank that swings 380 to 460 every two weeks.

  • Reef target: 380 to 450 ppm
  • Sweet spot: 410 to 440 ppm
  • Above 480 ppm: risk of abiotic precipitation
  • Daily swing should stay under 10 ppm

Calcium, alkalinity, and magnesium move together

This is the single most important concept in reef chemistry. Calcium and alkalinity are consumed in a fixed ratio when corals build skeleton — roughly 2 ppm of calcium for every 1 dKH of alkalinity. If you dose them in that ratio, both stay stable. If you dose them out of ratio, one drifts up while the other drifts down.

Magnesium is the third leg. It prevents calcium and alkalinity from precipitating out together. If magnesium drops below ~1250 ppm, your alkalinity and calcium will mysteriously refuse to climb no matter how much you dose. Fix magnesium first; then alkalinity and calcium will respond to dosing again.

  • Calcium target: 380–450 ppm
  • Alkalinity target: 7.5–9.5 dKH
  • Magnesium target: 1280–1400 ppm
  • Two-part dosing keeps Ca and Alk balanced (A = calcium, B = alkalinity)
  • Low magnesium blocks calcium and alkalinity dosing from working — always check Mg first

How to test calcium

Calcium test kits are generally less prone to user error than alkalinity titrations. Recommended:

  • Salifert Calcium Test — accurate, easy to read, widely available
  • Red Sea Foundation A Test — pairs naturally with Foundation B alkalinity kit
  • Hanna HI758 Calcium Checker — colorimeter for users who prefer digital reads
  • ICP labs (Aquaforest, ATI, Fauna Marin, Reef Moonshiner, Reef Zlements, Triton, Tropic Marin) — best cross-check

How to raise calcium

Calcium responds to dosing more slowly than alkalinity. Raising from 380 to 420 ppm typically takes 3 to 5 days of consistent dosing.

  • Two-part A (calcium chloride) — most common method. 1 mL per 100 L raises Ca by roughly 2 ppm with standard concentration solutions
  • Calcium reactor — dissolves calcium carbonate media, supplies calcium and alkalinity in 1:1 ratio
  • Kalkwasser — supplies calcium and alkalinity together, also raises pH
  • Aquaforest, Red Sea, Tropic Marin, ESV — all sell complete two-part programs

How to lower calcium

Calcium is hard to actively lower. The standard approach: stop dosing, let coral consumption draw it down, do partial water changes with a salt mix that lands near your target. Most calcium "high" issues resolve themselves in a week.

How Reef Trak tracks calcium

Reef Trak logs calcium readings from any test kit, supports the Hanna HI758 with automatic ppm conversion, and imports ICP results from 7 labs. The dosing calculator handles two-part calcium math automatically for any product (Red Sea Foundation A, BRS Pharma-grade, ESV, etc.). Multi-week trend charts let you see drift before corals notice.