A koi pond isn't just a body of water—it's a closed, living ecosystem. The quality of the water determines the health, growth, and color of the koi. But what happens beneath the surface? What role do microorganisms, chemical parameters, and the physical dynamics of the water play?
The bacteria – silent architects of balance
Billions of microorganisms live in every pond. Nitrifying bacteria, which drive the nitrogen cycle, are particularly important:
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Ammonium (NH₄⁺) is produced by fish feces and food residues.
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Nitrosomonas bacteria convert ammonium into nitrite (NO₂⁻) – a toxic intermediate.
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Nitrobacter bacteria transform nitrite into nitrate (NO₃⁻), which is significantly less toxic.
These processes require oxygen and stable conditions. Without adequate ventilation or if the biology is disrupted, dangerous concentrations can occur—especially at high temperatures and pH values, where ammonium converts to toxic ammonia (NH₃).
Chemical parameters – the invisible guardians
The most important water values in a koi pond are:
| parameter | Ideal range | Meaning |
|---|---|---|
| PH value | 7.2 – 7.6 | Controls biological processes, influences ammonia formation |
| Total hardness (dH) | 10 – 15 °dH | Mineral content, important for cell function and osmosis |
| Carbonate hardness (KH) | ≥ 5 – 8 °dH | Buffer against pH fluctuations, protects biological stability |
| Nitrite (NO₂⁻) | < 0.2 mg/l | Highly toxic to koi, inhibits oxygen uptake |
| Nitrate (NO₃⁻) | < 50 mg/l | Nutrient for plants, growth-inhibiting in high quantities |
| Ammonium/Ammonia | < 0.1 mg/l | Dependent on pH and temperature – potentially fatal |
Sources: Datz, Aquaculture Technology; Koi Organisation Germany eV
The natural water cycle in the closed pond system
In a closed pond system, there is no natural inflow or outflow. Therefore, the cycle must be artificially stabilized:
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Fish and plants produce organic matter.
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Bacteria convert these into soluble compounds.
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Filter technology removes solids and supports biological purification.
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Water movement ensures oxygenation, prevents stratification and promotes the distribution of nutrients and microorganisms.
Without movement, anaerobic zones develop, where decay and toxic gases like hydrogen sulfide can develop. So, flow isn't just aesthetic—it's vital.
Visual Insights: Molecular Processes and Pond Dynamics
I have prepared a suitable graphic for you that shows:
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The nitrogen cycle at the molecular level (NH₄⁺ → NO₂⁻ → NO₃⁻)
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The chemical conversion of ammonium to ammonia depending on pH and temperature
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The role of water movement in oxygen transport and the prevention of stratification
-->This visualization is ideal for brochures, training courses or websites with scientific standards and emotional depth.
Conclusion: Control is care – movement is life
A koi pond is a sensitive system. Knowing its water parameters, understanding its biology, and taking its physical dynamics into account not only protects your fish, but also the aesthetics and functionality of the entire system.
The combination of biological filtration, chemical stability and targeted water movement is the key to a pond that not only works – but inspires.