Magnesium (Mg)

Magnesium stabilises the balance between calcium and KH preventing spontaneous precipitation. Without adequate magnesium the Balling method does not work.

Detailed explanation

Magnesium is the third pillar of reef chemistry together with calcium and KH. Its main role is to prevent calcium and carbonates from precipitating spontaneously from the water, forming what aquarists call calcium snow.

In a well-populated marine aquarium the consumption of magnesium is slow but constant. Coralline algae absorb a lot of it. Some soft corals use it for spicules. The calcareous algae in the refugium also consume it.

The important thing is that magnesium should always be checked before correcting calcium or KH. If magnesium is low any attempt to raise calcium and KH ends in precipitation. Wasted money.

For freshwater and pond aquariums, magnesium contributes to GH and is needed by plants for photosynthesis. Red plants in particular require magnesium to produce pigments.

Reference values by aquarium type

How it is measured

Salifert drop test (the most used in marine aquariums). ICP analysis for precision. In freshwater aquariums it is rarely measured separately from GH.

What happens when it is too low

Calcium precipitation and KH. Calcium snow. Pumps that become encrusted. Impossible to keep the other parameters stable.

What happens when it is too high

Above 1500 ppm can inhibit coral calcification. Rarely dangerous but indicates a dosing problem.

Related terms