Recurring Koi Deaths: Water Chemistry Investigation

Caribbean pond (650 gal) lost all 16-inch koi over weeks—females full of eggs. Subsequent restocking: koi, tinfoil barbs, goldfish die; plecos, silver dollars, clown loaches, angels survive. Multiple water changes didn't stop losses.

Diagnostic Steps

1. Comprehensive water test: pH, hardness, alkalinity, ammonia, nitrite, nitrate, phosphate, copper, iron, oxygen, chlorine, salt, pesticides. Test both tap and pond water.
2. Overcrowding history: 16-inch koi in 650 gal = severe overcrowding → ammonia/nitrite spikes, low oxygen.
3. Egg binding: Possible but less likely with males present.
4. Tap water toxin: Consistent losses across species suggest source water issue (copper, chlorine, pesticide).
5. Aeration/filtration adequacy: Large fish need high oxygen; plecos gulp air, tolerate low O₂.

Immediate Actions

• Use dechlorinator with every water addition.
• Add pond salt at 0.05%.
• Increase aeration.
• Stop adding fish; stabilize current survivors.

Warm climate allows tropicals—stick with species that thrive.