Why Did My Pond Water Turn Black Overnight?

A pond owner reports their garden pond water turned black overnight, killing one koi. The same phenomenon occurred 10 miles away at a friend's pond, where half the koi perished. Both ponds required multiple drainings before clearing. The friend suspects aircraft fuel dumping, but the ponds had been stable for four years with no prior issues.

Expert Analysis: Weather-Induced Oxygen Crash

This sudden blackening is likely due to a dramatic weather shift—specifically, a rapid transition from calm conditions to intense thunderstorms. Such events cause a sharp drop in dissolved oxygen levels, suffocating fish and killing algae. As the algae die, they decompose and turn the water dark brown or black.

Key Questions to Diagnose

• Was there a sudden weather change (storms, temperature swing) in the days prior?
• Did the fish show distress signs—gasping at the surface, flashing, or jumping?
• What are the current water parameters: pH, ammonia, nitrite?

Testing water quality immediately after such an event is critical. While fuel dumping is theoretically possible, a weather-driven oxygen crash is far more common and fits the simultaneous occurrence at two locations.

Recommendation: Monitor weather forecasts, ensure robust aeration, and test water parameters regularly to catch oxygen sags early.