Potassium in the marine aquarium: role, ideal value, and correction

Biological & chemical role

Potassium is involved in many vital processes within the coral. It functions as a transport element for certain channel proteins, facilitating the passage of molecules across cell membranes. This function is crucial for tissue nutrition supply and intercellular communication within the coral colony.

In zooxanthellae, potassium participates directly in glucose synthesis during photosynthesis. These symbiotic algae, which provide most of the coral’s energy, depend on a stable potassium concentration to maintain their metabolism. A deficiency therefore affects not only the coral itself, but also its ability to produce energy through photosynthesis.

Potassium also plays a role in the formation of chromoproteins, the pigments responsible for pink, purple, and red shades in many corals. It is also incorporated into the calcareous skeleton and is involved in the metabolism of bacterial biofilms, as well as in many essential enzymatic reactions. This versatility makes it an element whose absence quickly shows through multiple symptoms.

Reference values and interpretation

• General target range: 380 to 420 mg/l, with an optimum between 400 and 420 mg/l.
• Relationship with calcium: potassium should ideally be about 20 mg/l below calcium (rule: Ca - 20 mg/l).
• Sensitivity threshold: a reduction of only 5% can already impact growth and coloration, especially in sensitive species.
• Critical low threshold: below 380 mg/l, deficiency signs gradually appear with slower growth and fading.
• Critical high threshold for corals: above 700 mg/l, tissue spots and small detachments may appear, with visible darkening and slower growth.
• Critical threshold for invertebrates: shrimp become sensitive from 500 mg/l and may suffer severe damage or die.

Measurement, reliability, and monitoring

Potassium can be measured with reliable home colorimetric tests, allowing regular monitoring without systematically relying on ICP analyses. Regular measurement is essential because consumption varies greatly from one system to another and evolves with modifications to the tank (addition of filter media, livestock changes, etc.).

It is recommended to test potassium at least once a month in stable tanks, and more frequently during phases of intense growth or after changes to the filtration system. Monitoring makes it possible to establish the consumption profile specific to each tank and to adjust dosing accordingly, thus avoiding progressive deficiencies that often go unnoticed until symptoms become obvious.

Interactions and common causes of variation

• Use of zeolites: these filter media actively consume potassium, significantly increasing supplementation needs.
• Ceramics and artificial decorations: some porous materials can adsorb potassium or alter its availability.
• Active bacterial cultures: bacteria use potassium in their metabolism, creating additional demand in heavily supplemented tanks.
• Intense coral growth: fast-growing species such as Montipora, Seriatopora, and Acropora consume substantial amounts.
• Nutritional imbalance: low phosphate combined with insufficient potassium amplifies deficiency symptoms.
• Salt quality: not all salts provide natural potassium concentrations; some require supplementation from the start.

Possible signs of imbalance

• Potassium too low:
  • General fading, gray or washed-out tones
  • Loss of intensity in vivid colors (pink, purple, red)
  • Slower growth or complete growth stop, especially in Montipora and Seriatopora
  • Absence of white growth margins in plating corals
  • Tissue loss in shaded areas
  • Sudden tissue loss in Euphyllia and other LPS
  • Poor nutrient conversion, potentially leading to elevated nitrate and phosphate levels
  • Dull tissues in Acropora valida, growth tips lacking purple coloration
• Potassium too high:
  • Visible darkening of colors
  • Slower growth
  • Tissue spots with small localized detachments (not from the base)
  • Burnt growth tips if iodine is simultaneously too low
  • Severe stress or mortality in shrimp from 500 mg/l onward

Key takeaway

Potassium is a macro-element requiring individualized management. Unlike calcium or magnesium, which are easily stabilized through water changes, potassium requires active monitoring and dosing adapted to each system. Indicator species such as Acropora valida and red plating Montipora reveal deficiencies quickly. In a balanced tank with regular water changes and good-quality salt, supplementation often remains moderate, but it becomes essential in systems using zeolites or housing a high density of fast-growing corals.