Pond Fish Kill After Runoff: Oxygen Crash and Toxins

Heavy runoff turned pond dark; bait fish, carp, bass, catfish died; tilapia and perch survived. Oxygen dropped to 2.02 mg/L. Robyn explains:

• Runoff brings silt, pesticides, oils, low-pH rainwater.
• Oxygen below 4 ppm is dangerous; 2 ppm is lethal for most species.
• Tilapia/perch tolerate lower oxygen.
• Dark color from soil/tannins settles out if natural.
• If toxins entered, they persist—carbon impractical for large ponds.

Install aeration; see farm pond suppliers for large-scale solutions.