Algae growth in a fish pond has both benefits and risks. The key lies in controlling its growth level. Moderate algae (such as a thin layer covering the bottom of the pond) can absorb harmful substances produced by excess fish waste and uneaten feed, increase oxygen through photosynthesis, and provide koi with natural plant-based supplementary food. Additionally, algae can suppress the outbreak of harmful algae and stabilize water quality.

However, excessive growth (thickness exceeding 0.5 cm or large floating patches) can also cause harm. For example, algae respiration at night consumes large amounts of oxygen, potentially causing hypoxia in koi. Long, filamentous algae can entangle fins and damage the fish's body, and decaying algae release toxins that pollute the water.

It is recommended to maintain a balance by adjusting shading (30%–50% shade), feeding amounts, and water change frequency. Regular observation and gentle manual removal, such as brushing or skimming, can prevent overgrowth without harming the pond ecosystem.

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I. Benefits of Algae

1. Natural Purifier
   Absorbs ammonia and nitrites, reduces pollution from fish waste and leftover feed, and stabilizes water quality.
2. Oxygenation Aid
   Releases oxygen through photosynthesis during the day, increasing dissolved oxygen levels in the water.
3. Ecological Barrier
   Suppresses the outbreak of harmful algae (such as blue-green algae) and helps maintain ecological balance.
4. Health Supplement for Koi
   Provides dietary fiber and trace elements, aiding digestion and enhancing coloration.

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II. Harm of Algae (When Excessive)

1. Oxygen Consumption at Night
   Respiration at night consumes large amounts of oxygen, which can easily cause hypoxia in koi, leading to surface gasping.
2. Entanglement Risk
   Long, filamentous algae can entangle fins, affecting swimming and even damaging the fish's body surface.
3. Decomposition Pollution
   Decaying algae release toxins like hydrogen sulfide, worsening water quality.
4. Clogging Filtration
   Shed algae can clog water pumps and filter media, reducing filtration efficiency.

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III. Scientific Management Recommendations

1. Control Growth Amount
   • Ideal state: A thin layer covering the pond bottom (≤0.3 cm), appearing bright green.
   • Warning signs: Thickness >0.5 cm, large floating patches, or turbid water.
2. Ecological Control Methods
   • Shading: Use shade nets to control 30%–50% of light exposure.
   • Biological control: Introduce black-shell shrimp (10–15 per ton of water) to graze on algae.
   • Competition: Plant aquatic plants like water lilies to absorb excess nutrients.
3. Timing for Physical Cleaning
   • Use a long-handled brush weekly to clean sun-exposed pond walls.
   • Promptly remove floating algae with a fine-mesh net.

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IV. Important Notes

• Avoid chemical algaecides (they harm koi and the nitrification system).
• Increase cleaning efforts after heavy rain (rainwater introduces nitrogen and phosphorus, promoting algae growth).
• Retain some algae in winter to help with insulation and provide natural food.