Potassium (K)

Potassium is an often overlooked trace element in marine aquariums. It contributes to the colour of corals, especially intense reds and purples.

Detailed explanation

Potassium naturally occurs in sea water at approximately 380-400 ppm. In the aquarium it tends to decrease over time because the coralline algae and some corals absorb it.

Potassium deficiency is subtle: corals do not die but lose their bright colours. Reds turn brown. The purples fade. Many aquarists blame lighting or nutrients when in reality potassium is lacking.

Supplementation is done with potassium chloride or potassium sulphate. Some premium Balling systems include potassium. Alternatively it is dosed separately.

In planted freshwater aquariums, potassium is a fundamental macronutrient. Plants absorb it in large quantities. The deficiency manifests itself as holes in old leaves (plants move potassium from old leaves to new ones). It is integrated with specific fertilisers (Seachem Flourish Potassium, ADA Brighty K).

Reference values by aquarium type

How it is measured

ICP Analysis for marine aquariums (no reliable drop test). For freshwater aquariums JBL K tests are available but not widespread.

What happens when it is too low

Marine: corals with faded colours, reds turning brown. Freshwater: Holes in old leaves of plants.

What happens when it is too high

Rarely problematic. Above 500 ppm in marine aquariums it may interfere with other processes.

Related terms