Understanding the relationship between calcium, alkalinity (KH), and magnesium for coral skeletal formation, and how to maintain proper balance through dosing, testing frequency, and calcium reactor use.
Key Takeaways
Understanding the relationship between calcium, alkalinity (KH), and magnesium for coral skeletal formation, and how to maintain proper balance through dosing, testing frequency, and calcium reactor use.
Corals (particularly SPS and LPS) build their calcium carbonate skeletons using calcium (Ca) and alkalinity (KH) from the water. Supporting this "calcification" process are three pillars of reef tank management: calcium, alkalinity (KH), and magnesium (Mg).
Since these three elements interact with one another, adjusting just one will not achieve stability. Managing them while maintaining balance is the key to long-term tank success.
| Parameter | Target Value | Measurement Frequency | |---|---|---| | Calcium (Ca) | 380–450 ppm | Once per week | | Alkalinity (KH) | 7–11 dKH | Once to twice per week | | Magnesium (Mg) | 1,250–1,350 ppm | Once to twice per month |
KH is the most variable parameter, so developing a habit of measuring twice weekly provides peace of mind. When the calcium to KH ratio becomes unbalanced, precipitation can occur, so awareness of the Ca:KH ratio is important (Ca 400 ppm ≈ KH 8–9 dKH is a useful baseline).
The simplest method is two-part dosing such as "Aart additives (Part 1 & 2)". Calcium and alkalinity components (sodium bicarbonate, etc.) are added separately.
Important: Adding both solutions simultaneously or in the same location causes precipitation. Always add them in different locations with time gaps between doses. Daily dosing of small amounts is ideal.
Weekly 10–15% water changes using high-quality synthetic seawater containing Ca and KH serve to replenish these parameters. In many cases, consistent water changes alone can adequately maintain Ca and KH levels, making it the most recommended method for beginners.
A calcium reactor dissolves coral skeleton material (aragonite) using CO2 and returns the reactor water to the tank. It is particularly effective in SPS-dominated tanks and large systems.
Typical symptoms of Mg deficiency are "additives don't raise Ca and KH" and "white precipitate appears". When Mg is low (below 1,200 ppm), precipitation of Ca and KH is promoted, and supplementation becomes ineffective no matter how much is added.
To supplement Mg, use magnesium chloride or commercially available magnesium additives. Adding large amounts at once affects specific gravity and other parameters, so gradually increase by 50–100 ppm per day for safety.
In addition to monthly test kit measurements, ICP water analysis performed 1–2 times yearly provides comprehensive data including trace elements (strontium, iodine, vanadium, etc.). ICP services are available online starting from a few thousand yen and can be used as a "health checkup" for your tank.
Managing Ca, KH, and Mg in coral tanks begins by establishing a habit of the "measure → dose → measure" cycle. Beginners should focus on water changes, and as coral numbers increase, consider introducing two-part additives or calcium reactors. Maintaining the balance of these three elements is the shortcut to a beautiful and healthy reef tank.
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