Chlorine Overdose from Forgotten Hose: Saving Koi

Hose left running overnight—pond now has chlorinated tap water. Fish distressed. Husband fears all will die. Is it hopeless?

Chlorine Toxicity

• Municipal water: 0.5–2 ppm chlorine/chloramine. Lethal to fish at 0.1 ppm.
• Damages gill epithelium irreversibly—impairs oxygen uptake long-term.
• Signs: gasping, erratic swimming, mucus hypersecretion, death within hours–days.

Emergency Protocol

1. Dechlorinate NOW: Add sodium thiosulfate-based conditioner (e.g., Seachem Prime, API Pond Stress Coat) at **double dose** for the *entire pond volume*.
2. Maximize Aeration: Air stones, waterfall, spray bar—drive off residual chlorine gas.
3. Salt Support: Add non-iodized pond salt to 0.2% (2 lbs/100 gal). Reduces osmotic stress, supports slime coat.
4. Stabilize pH: Test and buffer to 7.0–7.8. Prevents ammonia toxicity swings.
5. Zero Feeding: Until fish behave normally 48+ hours.
6. Monitor Ammonia: Damaged gills excrete poorly—ammonia spikes likely. Daily 10–20% water changes (dechlorinated!) if NH₃ >0.25 ppm.

Prognosis

Fish with severe gill burns may die despite treatment. Survivors often recover fully but remain sensitive. Learn from this: install an automatic shutoff timer or flow sensor on fill lines.