Algaecide Overdose: Saving Koi from Chemical Poisoning

Recognizing Algaecide Toxicity

Red blood streaks on fins and a pinkish body hue are classic signs of chemical burn from an overdose of algaecide (often copper-based or quaternary ammonium compounds). The treatment also likely crashed the pH, compounding the stress.

Immediate Rescue Steps

1. Massive water change: 50–75% immediately, using dechlorinated, temperature-matched water.
2. Add activated carbon to the filter to adsorb remaining chemical.
3. Stop all algaecide treatments. The "every 3 days" regimen with 20% changes was too aggressive.
4. Aerate heavily: Air stones, waterfall, or pump output directed at the surface.
5. Monitor pH, ammonia, nitrite daily for the next week.

Long-Term Algae Management

Green water is best controlled by shading (plants), nutrient reduction (bacteria, barley straw), and UV clarification—not repeated algaecide dosing. Your 8–9-year-old koi deserve a gentler approach.