During the day, algae produce oxygen through photosynthesis. However, at night they switch to respiration, consuming large amounts of dissolved oxygen in the water. When algae overgrow (water turns dense green, clarity drops), their nighttime oxygen consumption can be 2–3 times that of koi, causing dissolved oxygen to fall to dangerous levels (<3 mg/L) between 3:00 a.m. and 5:00 a.m., leading to koi gasping at the surface or even suffocation.
Key Warning Signs:
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Water is thick green, visibility <30 cm.
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Koi frequently gasp at the surface and gather near water inlets or air stones during early morning hours.
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Dissolved‑oxygen meters show a sharp drop in readings overnight.
Emergency Response:
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Immediately turn on all air pumps at full power and add oxygen tablets.
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Change 20%–30% of the water (new water should be aerated in advance).
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Stop feeding to reduce oxygen demand.
Long‑Term Prevention:
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Control light: Cover 30%–50% of the water surface with shade cloth.
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Reduce nutrients: Feed appropriately and clean waste regularly.
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Biological competition: Grow floating plants like duckweed or water hyacinth.
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Mechanical assistance: Use UV sterilizers or algae filters.
Special Note:
Severe oxygen depletion can cause algae to die off. As dead algae decompose, they consume even more oxygen and release toxins, creating a vicious cycle: oxygen drop → algae death → further oxygen depletion.
