Mysterious Fish Deaths in a Well-Filtered Pond: Troubleshooting Guide

When Water Looks Good but Fish Keep Dying

A 900-liter pond with excellent filtration and aeration loses a fish every two weeks with no visible disease. This pattern suggests a chronic, invisible water quality issue or a pathogen not apparent on external exam.

Critical Tests (Must Have)

• pH: Ideal 7.0–8.0; drift outside this range stresses fish.
• KH (Alkalinity): > 100 ppm to buffer pH.
• GH (Hardness): Match species needs; very hard water can harm some fish.
• Ammonia & Nitrite: Must be 0 ppm.
• Nitrate: < 40 ppm (lower for sensitive species).
• Temperature: Record daily; extremes cause stress.

Common Hidden Culprits

• pH crash: Low KH allows pH to plummet overnight, killing fish silently.
• Heavy metals (copper, zinc): From plumbing, algaecides, or runoff. Test with a comprehensive kit.
• Chloramine: Tap water treated with chloramine requires specific neutralizers.
• Parasites (gill flukes, Costia): Cause respiratory distress without external signs. Require microscopic scrapes.
• Bacterial septicemia: Internal infection; fish die suddenly. Often secondary to stress.

Action Plan

1. Test full parameter set at different times of day.
2. Perform a 30% water change with dechlorinator.
3. Add pond salt (0.1% = 1 kg/1000 L) and MelaFix as supportive care.
4. Contact a local koi club or fish vet for microscopic diagnosis.
5. Review all inputs: food, top-off water, treatments, runoff.

Without test data, you're guessing. Invest in a liquid test kit (API, NT Labs) and log results.