Coral Tank KH & Calcium Management — Additive Usage Guide
Managing alkalinity, calcium, and magnesium in coral tanks with two-part dosing, kalkwasser, and calcium reactors.
Key Takeaways
Managing alkalinity, calcium, and magnesium in coral tanks with two-part dosing, kalkwasser, and calcium reactors.
In stone coral (LPS and SPS) husbandry, managing KH (alkalinity) and calcium is among the most critical aspects of water quality management. Corals absorb calcium carbonate from seawater to build their skeletons, so when these values decline, growth stops, and in worst cases, skeletons may begin to dissolve. This article explains the relationship between KH, calcium, and magnesium, and covers specific supplementation and management methods.
The KH, Calcium, and Magnesium Relationship
These three elements are closely interrelated, and managing only one is ineffective.
KH (carbonate hardness) measures the total carbonate and bicarbonate ions in seawater. Corals combine these ions with calcium ions to build calcium carbonate (CaCO3) skeletons. Natural seawater KH is about 7 dKH (125 ppm), with 7-11 dKH recommended for aquariums. For active SPS growth, 8-9 dKH is ideal.
Calcium (Ca) is the direct building material that combines with KH to form skeletons. Natural seawater calcium is about 420 ppm; maintain 400-450 ppm in aquariums. Calcium deficiency causes coral tips to become thin and white, with dramatically slowed growth.
Magnesium (Mg) acts as a "mediator" preventing calcium carbonate precipitation. Without sufficient magnesium, added calcium and KH spontaneously combine and precipitate in the water, making it impossible to raise either value. Natural seawater is about 1,280 ppm; maintain 1,250-1,350 ppm in aquariums. If KH and calcium refuse to rise, check magnesium first.
Measuring Consumption Rates
To use additives correctly, first determine how much KH and calcium your tank consumes.
The method is simple: measure KH and calcium immediately after a water change, then measure again 24 hours later. The difference is your daily consumption. For example, if morning KH is 8.5 dKH and the next morning reads 7.8 dKH, daily consumption is 0.7 dKH.
Soft coral tanks consume very little, often manageable with weekly water changes alone. LPS tanks have moderate consumption. SPS tanks, especially those heavy with Acropora, can consume over 1 dKH per day.