Coral Aquarium Water Quality Management Guide | Calcium, Alkalinity, and Magnesium Supplementation and Measurement
A detailed guide to the role and target values of calcium, alkalinity (KH), and magnesium essential for coral skeleton formation, measurement methods for each element, and how to choose between two-part additives, kalkwasser, and calcium reactors.
Key Takeaways
A detailed guide to the role and target values of calcium, alkalinity (KH), and magnesium essential for coral skeleton formation, measurement methods for each element, and how to choose between two-part additives, kalkwasser, and calcium reactors.
Related Species
Fundamentals of the Three Essential Water Quality Elements for Coral Keeping
To maintain healthy corals, the three elements of calcium (Ca), alkalinity (KH), and magnesium (Mg) have an inseparable relationship. These three are components that directly support coral skeleton formation (calcification), and if any one becomes imbalanced, the coral will close its polyps, fade in color, and in the worst case begin to dissolve.
The target values for each component in natural seawater are approximately as follows.
- Calcium: 380–420 mg/L
- Alkalinity: 8–10 dKH (2.86–3.57 meq/L)
- Magnesium: 1250–1350 mg/L
Although the three elements appear to be independent, in reality they interact with each other. When magnesium decreases, calcium and alkalinity become more prone to precipitation, creating a situation where values do not rise no matter how much is added. Restoring magnesium to its proper range is often the first step in water quality management.
Measurement Frequency and Selection of Reliable Test Reagents
The precision of water quality management is directly linked to measurement precision. Inexpensive test reagents make color discrimination difficult, and even experienced aquarists can confuse 5 dKH with 7 dKH. For those focusing on SPS (demanding corals), colorimeters from Hanna Instruments, or titration reagents from Salifert and Red Sea are well-regarded for their reliability.
General guidelines for measurement frequency are as follows.
- Initial setup / immediately after adding specimens: daily
- When the tank is stable: 1–2 times per week
- Tanks with many large LPS or SPS: 2–3 times per week
Always use sample water from the same time of day and the same location. Water taken immediately after the lights turn on and water taken one hour after the lights turn off may show subtle differences in alkalinity due to pH fluctuations caused by photosynthesis. It is important to establish a repeatable measurement protocol.
