Sudden Large Koi Death in Heat: Oxygen, Chlorine, and Gill Damage

Florida, high 90s. One koi gasping → recovered with fresh water + aerators. Next day, different 5+ lb koi seizing, beaching, dies. Pond 2500 gal, 9 koi. Cause?

Chronic Gill Damage from Chlorine

"Fresh water" added yesterday = likely tap water. If not fully dechlorinated, chlorine/chloramine burns gills. Damaged gills = reduced O2 uptake. Heatwave pushes demand over supply → sudden death. The first koi survived (mild damage); the second had worse damage or less reserve.

Immediate Actions

1. Test Ammonia/Nitrite: Must be 0. If not, 33% daily changes + Prime.
2. Large Aeration: 2+ large air stones at depth. Run 24/7.
3. ALWAYS Dechlorinate: Every drop of tap water. Use double dose if unsure.
4. Shade Pond: 50% shade cloth. Lowers temp significantly.
5. Stop Feeding: Reduces metabolic O2 demand and ammonia.

Long-Term

2500 gal for 9 large koi is tight. Plan for 3000+ gal or reduce stock. Install permanent aeration + dechlorinator dosing system (e.g., inline carbon block).