Short Answer:
A healthy pond should have 5–8 mg/L of dissolved oxygen, with at least 3 mg/L minimum.

Detailed Answer:

• 5–8 mg/L → Ideal for fish and aerobic bacteria
• 3–5 mg/L → Stressful for fish
• Below 3 mg/L → Risk of fish kills

When oxygen drops:

• Hot summer nights
• Cloudy days (less photosynthesis)
• After algae die-offs

Best prevention:
Consistent aeration, especially overnight during warm weather.