Coral Tank Water Chemistry Guide — Calcium, Alkalinity & Magnesium Dosing and Testing | ブリちょく
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Coral Tank Water Chemistry Guide — Calcium, Alkalinity & Magnesium Dosing and Testing
Complete guide to coral tank water chemistry: the role and target values of calcium, alkalinity (KH), and magnesium; how to test them; and choosing between two-part dosing, kalkwasser, and calcium reactors.
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Complete guide to coral tank water chemistry: the role and target values of calcium, alkalinity (KH), and magnesium; how to test them; and choosing between two-part dosing, kalkwasser, and calcium reactors.
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Coral Tank Water Quality Management Guide | Adding and Measuring Calcium, Alkalinity, and Magnesium
For coral keeping (especially LPS and SPS), the centerpiece of water quality management consists of three essential elements: "Calcium (Ca)," "Alkalinity (KH/Alkalinity)," and "Magnesium (Mg)". These elements are crucial for coral skeletal formation, and their deficiency halts coral growth, ultimately leading to bleaching and death. This article provides a detailed explanation of the roles, target values, measurement, and addition methods for these three essential elements.
Coral Skeletal Formation and Water Quality Relationship
Corals synthesize calcium carbonate (CaCO₃) from seawater calcium and carbonates (supplied by alkalinity) to build their skeleton. Magnesium is necessary as a catalyst for this skeletal formation.
In other words, calcium and alkalinity (a source of bicarbonate ions) are consumed simultaneously. As corals grow, these elements decline, necessitating replenishment.
Target Values for the Three Essential Elements
Element
Natural Seawater Value
Coral Tank Target
Measurement Method
Calcium (Ca)
420 mg/L
380–450 mg/L
Test kit • ICP
Alkalinity (KH/dKH)
8–9 dKH
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7–12 dKH
Test kit
Magnesium (Mg)
1285 mg/L
1250–1400 mg/L
Test kit • ICP
Note: KH (carbonate hardness) and alkalinity are essentially the same parameter expressed in different units. 1 dKH ≈ 17.9 mg/L CaCO₃ equivalent.
Why Mg (Magnesium) is Important
When magnesium is deficient, calcium is less efficiently incorporated into the coral skeleton, and coralline algae may excessively accumulate on glass and rocks.
Symptoms of Low Mg:
- Slowed coral growth
- Rapid decline even after adding Ca and KH
- Excessive coralline algae accumulation on glass surfaces
Advantages:
- Low equipment cost (optional automatic dosing pumps)
- Relatively easy to begin
Disadvantages:
- Consumable costs add up (expensive for high-consumption tanks)
- Trace elements other than Ca and KH must be added separately
Calculation of Dosing Amount:
- First measure current values
- Calculate the difference from target values
- Determine dosing amount based on this calculation
- Re-measure 24–48 hours after dosing and adjust
3. Calcium Reactor (Most Professional Method)
An apparatus that dissolves coralline limestone by bubbling CO₂ through it, returning the dissolved components to the tank.
Advantages:
- Lowest long-term operating costs
- Replenishes Ca and KH simultaneously in balanced proportions
- Also releases trace elements
Causes: Accelerated coral growth, insufficient dosing, reduced water change frequency
Solutions:
1. Increase dosing amount (add small amounts multiple times rather than one large dose)
2. If dropping rapidly, be cautious about raising by more than 0.5–1 dKH at once
Caution: Raising KH by more than 2 dKH in one day can damage corals.
Solutions: Add only the low element to balance it, then reassess dosing balance afterward.
Low Mg
Solution: Add commercial Mg additives gradually (raising Mg by approximately 100 mg/L over 1–2 days). Be careful about rapid changes, as they can damage corals.
Summary
The essence of coral tank water quality management is maintaining Ca, KH, and Mg at appropriate levels. By repeating the cycle of measurement → addition → re-measurement, you can discover the dosing amount suited to your tank's consumption rate.
During the soft coral stage, water changes alone are sufficient, but when stepping up to LPS and SPS, consider introducing a supplementation system. By combining periodic ICP testing, you can detect hard-to-see trace element issues early, maintaining a beautiful coral tank for the long term.
