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Reef Tank Calcium Calculator
Reef Tank Calcium Calculator — How Much CaCl₂ to Add
Calcium is the most consumed major element in a reef aquarium. Stony corals (LPS and SPS) extract calcium from the water to build their skeletons at a rate that depends on coral density, light, and growth speed. The natural seawater level is approximately 420 mg/L (ppm), and most reef systems thrive between 380–450 ppm. Levels below 360 ppm slow coral growth and bleaching becomes more likely; levels above 500 ppm can cause aragonite precipitation and a dangerous KH crash. When your ICP test or Alk/Ca test kit shows calcium is low, this calculator tells you exactly how many grams of calcium chloride to add. It accounts for the three common forms of CaCl₂ (anhydrous, dihydrate, hexahydrate), which have very different gram-weights per unit of actual calcium. Always dose slowly and re-test 24 hours later before adding more.
How to Use This Calculator
1
Test your tank calcium with a reliable test kit or ICP analysis and note the current value in ppm (mg/L).
2
Enter your target calcium level — 420–440 ppm is ideal for most mixed reef tanks.
3
Enter your total tank water volume in liters (exclude rock and substrate displacement — use about 75% of gross volume for a rough estimate).
4
Select the form of CaCl₂ you have. Check the bag — this matters significantly for correct dosing.
5
Dissolve the calculated amount in a cup of RODI water first, then add slowly to high-flow area. Re-test after 24 hours before dosing again.
Need reef supplements?
We stock Brightwell Aquatics, Two Little Fishies, and premium additives — shipped with your corals.
Frequently Asked Questions
What is the ideal calcium level for a reef tank?
Natural seawater contains approximately 420 mg/L of calcium. Most reef aquariums do well between 380–450 ppm. SPS-dominant tanks often target the higher end (430–450 ppm) because fast-growing acropora consumes calcium rapidly. LPS-dominant tanks can thrive at 380–420 ppm.
Why is my calcium dropping?
Corals consume calcium as they grow their skeletons. The rate depends on coral mass and growth speed. A heavily stocked SPS tank can consume 20–50 ppm of calcium per day. Regular dosing (two-part, Balling method, or a calcium reactor) is the only sustainable long-term solution.
Can I add too much calcium at once?
Yes. Adding large doses rapidly can cause calcium and alkalinity to temporarily go out of balance, leading to precipitation (white cloudiness) and a sudden drop in KH. If you need to raise calcium by more than 20 ppm, split the dose across 2–3 days.
What is the relationship between calcium and alkalinity?
Calcium and alkalinity must stay in balance. The ratio in natural seawater is approximately 2.8:1 (Ca ppm : KH dKH). If you raise calcium without raising alkalinity proportionally, or vice versa, you risk precipitation. Always test both parameters together.
