Calcium, Alkalinity and Magnesium in the Reef Tank

The 'big three' reef parameters for coral skeletal growth: what calcium, alkalinity and magnesium do, their target ranges, why they are interlinked, and how to keep them stable.

Overview

Calcium, alkalinity and magnesium are often called the reef aquarium's 'big three' because together they govern the calcification by which stony corals, coralline algae and many invertebrates build their calcium carbonate skeletons. Maintaining these parameters in the right ranges, and keeping them stable, is central to keeping a reef tank with hard corals. Reef-chemistry writers stress that stability and the balance between the three matter more than chasing an exact number.

Calcium

Calcium is one of the two main building blocks of coral skeleton. Natural seawater holds roughly 420 ppm of calcium, and reef aquaria are commonly kept in the range of about 380 to 450 ppm. Corals remove calcium from the water as they grow, so it must be replenished. Calcium cannot be considered alone, because it is consumed in step with carbonate (alkalinity).

Alkalinity (carbonate)

Alkalinity is a measure of the water's carbonate and bicarbonate, the carbonate half of the calcium carbonate skeleton, and it also buffers pH against swings. It is reported in degrees of carbonate hardness (dKH) or milliequivalents per litre (meq/L). Natural seawater is about 7 dKH (roughly 2.5 meq/L); reef aquaria are typically kept around 7 to 12 dKH (about 2.5 to 4.3 meq/L). Many reefkeepers hold the lower part of this range for stability.

Magnesium

Magnesium is the third most abundant ion in seawater and the reason calcium and alkalinity can stay elevated together without precipitating. Natural seawater holds roughly 1,280 to 1,300 ppm, and reef aquaria are commonly kept around 1,250 to 1,350 ppm. Magnesium interferes with the formation of calcium carbonate crystals, so it permits both calcium and carbonate to remain dissolved above the level they otherwise would.

Why they are interlinked

Corals build skeleton from calcium and carbonate in a fixed proportion, so the two are drawn down together; this is why balanced supplements that supply calcium and alkalinity in matching amounts are recommended. Magnesium sits over both, holding them in solution. If magnesium is substantially depleted, calcium carbonate tends to precipitate from the water, and it then becomes difficult to raise or hold calcium and alkalinity at all.

Testing and stability

• Test alkalinity most often, as it is consumed quickly and shifts first; calcium and magnesium can be tested less frequently.
• Aim for steady values rather than the highest number; sudden changes stress corals more than a slightly off but stable level.
• Check magnesium when calcium and alkalinity are hard to keep up, since low magnesium can be the hidden cause.
• Track daily consumption so dosing can be matched to what the corals actually use.