Sudden Goldfish Deaths in Mature Pond: Oxygen or New Fish?

2-year-old pond (3.5x2x1.2m) with waterfall, biofilter, UV, plants, 20 sarasa comets. Added 10 small sarasas to upper pond for 3-month acclimation. After hot spell, 7 large fish + 3 fry died. Water tests "fine." Added fountain + air pump. One more died next day. New fish in upper pond fine.

Likely Culprit: Nighttime Oxygen Crash

• Hot weather reduces oxygen solubility.
• Duckweed cover limits gas exchange.
• "Oxygenating" plants consume oxygen at night.
• Large fish have higher oxygen demand—die first.
• Upper pond has waterfall aeration—new fish survive.

Actions Taken (Good)

• Added fountain + air pump—increases surface agitation.
• Removed some duckweed (net it out).
• Run aeration 24/7.

Missing Test

Test dissolved oxygen at dawn. If low, that's the cause. Delayed deaths occur as weakened fish succumb.

Quarantine Lesson

3 months in upper pond isn't true quarantine (shared water). Always quarantine new fish in separate system.

If oxygen was the issue, losses should stop with sustained aeration.