The 2–5 AM danger: why night dissolved oxygen kills crops

Dissolved oxygen (DO) in a pond is highest in the late afternoon — when plankton have been photosynthesising all day — and lowest between about 2 and 5 AM, when there is no sunlight and everything in the pond is still consuming oxygen. On a heavily stocked pond with a dense plankton bloom, DO can fall below the safe 4 mg/L floor overnight and trigger sudden mortality before sunrise.

The warning signs build during the day: a very heavy or "crashing" bloom, low transparency, high feed loads, and warm still weather. If DO is already trending low in the evening reading, the night low will be worse.

Practical defence: run aerators through the night (especially 1–6 AM), avoid overfeeding when the bloom is heavy, exchange water if the bloom is crashing, and take an evening DO reading so you can predict the night low. AquaGrow flags a low-DO risk automatically from your daily logs so you can act the same evening.