1. Introduction
Importance of Oxygen in a Koi Pond
The oxygen content in a koi pond is one of the most important, yet often underestimated, parameters of the entire pond biology. While many koi keepers intensively monitor pH, nitrite, or ammonium, in practice, dissolved oxygen (O₂ = molecular oxygen) determines the stability of the entire system.
Oxygen simultaneously influences:
- the respiration of the koi
- the efficiency of the biofilter
- the decomposition of organic matter
- the immune system of the fish
- the biological stability of the water
An oxygen deficiency always affects the entire system and often leads to a rapid deterioration of water quality.
2. Importance of Oxygen for Koi
2.1 Physiological Function
Koi absorb oxygen through their gills. This is used in metabolism for cellular respiration.
→ Oxygen serves for energy production in the form of ATP (adenosine triphosphate, universal energy carrier of cells)
→ without ATP, no efficient energy production is possible
2.2 Effects of Oxygen Deficiency
With too low oxygen content:
- reduced activity of the fish
- decreased food intake
- slowed growth
- weakened immune system
- increased susceptibility to stress
This situation is particularly critical in summer, as oxygen demand simultaneously increases and solubility in water decreases.
3. Oxygen Requirement of the Biofilter
The biofilter is a biological system that breaks down pollutants.
3.1 Nitrogen Cycle
Food → Ammonium (NH₄⁺) → Nitrite (NO₂⁻) → Nitrate (NO₃⁻)
This process is carried out by nitrifying bacteria:
→ Nitrosomonas
→ Nitrobacter
→ Nitrospira
3.2 Oxygen Dependence
These bacteria work exclusively under aerobic conditions (with oxygen).
→ without oxygen:
- filter performance drops sharply
- toxic substances can accumulate
- the biological balance collapses
4. Oxygen Amount in a Koi Pond
4.1 Basic Principle
The amount of oxygen in the water is very low:
- Air: approx. 21% oxygen
- Water: approx. 6–10 mg/L dissolved oxygen
4.2 Example Calculation
Given:
- Pond volume: 20,000 liters
- Oxygen concentration: 8 mg/L
Calculation:
- 20,000 × 8 = 160,000 mg
- = 160 g oxygen in the entire pond
Conclusion:
→ The entire pond contains only a very small amount of stored oxygen.
5. Oxygen Consumption in a Koi Pond
5.1 Koi Consumption
Guideline from aquaculture:
- 250–600 mg O₂ (molecular oxygen) per kg fish per hour
- Average: approx. 400 mg/kg/h
5.2 Example Calculation
- 20 Koi
- Ø 4 kg
- Total biomass: 80 kg
Calculation:
- 80 × 400 mg = 32,000 mg/h
- = 32 g O₂ per hour
- = approx. 770 g O₂ per day (koi only)
5.3 Overall System
Additionally consumed by:
- Biofilter
- Microorganisms
- Organic decomposition
→ real total demand often 6–8 kg O₂ per day (for 20,000 L pond)
6. Influence of Temperature
With increasing temperature:
- the solubility of oxygen in water decreases
- the metabolism of the koi increases
- the activity of bacteria increases
Result:
→ less oxygen with simultaneously higher consumption
7. Oxygen Input into Water
Important:
→ air bubbles are not the primary mechanism of oxygen enrichment
Crucial are:
- surface movement
- water circulation
- contact between water and air
8. Efficiency of Different Oxygen Systems
Air Stones (dependent on depth)
- 0.5 m → 5–8 %
- 1.0 m → 8–12 %
- 1.5 m → 12–18 %
- 2.0 m → 18–25 %
- 2.5 m → 22–30 %
Waterfall
- 10–20 % Efficiency
Stream
- 15–25 % Efficiency
Venturi System
- 10–20 % Efficiency
Bottom Aeration
- 20–35 % Efficiency
Trickle Filter
- 40–70 % Efficiency (very high oxygen enrichment)
9. Influence of Depth on Compressor Pressure
9.1 Water Pressure
→ per 1 meter depth = approx. 0.1 bar pressure (Bar = unit for pressure)
| Depth | Pressure |
|---|---|
| 0.5 m | 0.05 bar |
| 1.0 m | 0.10 bar |
| 2.0 m | 0.20 bar |
9.2 Practical Effect
Example:
- Air stone at 2 m depth → 0.20 bar back pressure
- Compressor capacity: 0.35 bar
→ effectively usable: 0.15 bar
Conclusion:
→ a portion of the energy is used exclusively to overcome the water pressure
10. Practical Formula for Air Volume Calculation
10.1 Basic Formula
Air volume (L/min) = Pond volume (m³) × 1.0 + Number of Koi × 1.5
10.2 Example
- 20,000 Liters = 20 m³
- 20 Koi
→ 20 × 1.0 = 20 L/min
→ 20 × 1.5 = 30 L/min
Total:
→ 50 L/min air performance
10.3 Safety Reserve
→ × 1.3 recommended
Result:
→ approx. 65 L/min compressor performance
11. Oxygen Requirement of the Entire Pond
- 5,000 L → 1–2 kg O₂ per day
- 10,000 L → 3–5 kg O₂ per day
- 20,000 L → 6–8 kg O₂ per day
- 40,000 L → 10–16 kg O₂ per day
12. Oxygen Measurement in a Koi Pond
Methods:
- Drop tests (simple, but inaccurate)
- Digital measuring devices (precise, professional)
Measurement unit:
→ mg/L dissolved oxygen (O₂)
13. Signs of Oxygen Deficiency
- Koi standing at the surface
- rapid gill movement
- reduced activity
- refusal to feed
- poor filter performance
14. Conclusion
Oxygen is the central controlling factor in a koi pond. No other parameter simultaneously influences so many biological processes.
Crucial is not only the absolute quantity, but the interaction of:
- pond volume
- fish stock
- filter performance
- aeration system
- water depth and pressure conditions
A stable oxygen balance leads to:
- healthy koi
- stable water quality
- efficient biofilter
- significantly reduced risk of disease