I. Ecological Benefits of Algae

1. Natural Water Purifiers
   Through photosynthesis, they absorb ammonia and nitrites while releasing oxygen, helping to stabilize water quality.
2. Microbial Symbiosis
   Algal films provide surfaces for nitrifying bacteria to colonize, enhancing biological filtration.
3. Natural Supplementary Food for Koi
   Small amounts of algae offer plant fibers and trace elements to koi.
   

II. Warning Signs Requiring Intervention

1. Excessive Algae Growth
   • Pond walls are completely covered, affecting aesthetics.
   • Algae consume large amounts of oxygen at night, potentially causing koi to gasp at the surface due to hypoxia.
2. Abnormal Algae Die-Off
   • Water suddenly turns cloudy and emits a foul odor.
   • Large amounts of algal debris float on the surface.
3. Impact on Koi Health
   • Algal filaments entangle fins, restricting movement.
   • Thick algal films harbor parasites (e.g., anchor worms).

III. Scientific Management Recommendations

1. Light Algae – Retain Benefits
   • Maintain about 30% algae coverage on pond walls (thin, light green layer).
   • Regularly scrub sun-exposed areas while retaining algae on shaded surfaces.
2. Moderate Algae – Manual Control
   • Use a magnetic brush weekly to clean visible areas.
   • Introduce algae-eating fish (e.g., Siamese algae eaters), with 1–2 per ton of water.
3. Heavy Algae – Thorough Cleaning
   • Step 1: Remove algal layers from pond walls using an algae scraper.
   • Step 2: Change 30% of the water and add nitrifying bacteria.
   • Step 3: Adjust lighting (shade the pond or reduce light exposure time).
     

IV. Key Measures to Prevent Overgrowth

1. Control Light Exposure: Limit direct sunlight to ≤6 hours daily, or use aquatic plants for shading.
2. Reduce Nutrient Sources: Decrease feeding amounts and clean the filter compartment monthly.
3. Biological Competition: Grow higher plants like water lilies or water hyacinths to absorb excess nutrients.
4. Physical Interception: Install brush filters at the filter inlet to trap algal spores.

V. Important Notes

• Avoid chemical algaecides (they can harm koi gills and the nitrogen cycle).
• Strengthen algae cleaning after heavy rain (rainwater carries nitrogen and phosphorus that promote algae growth).
• Retain some brown algae in winter to aid insulation and provide natural food.