Diagnosing a Tench Floating at the Surface After Heavy Rain

Potential Causes

• Low Oxygen: Heavy rain can stratify the pond or wash in organic matter that decays, consuming oxygen. Tench are sensitive to low O₂.
• pH/Hardness Crash: Rain is soft and acidic. Large influx drops pond hardness and pH, stressing fish.
• General Illness: Parasites, bacteria, viruses, or internal issues cause lethargy and surface hovering.

Immediate Actions

1. Test pH, KH (hardness), ammonia, nitrite, and oxygen.
2. Increase aeration (air stone, waterfall, pump).
3. Add pond salt at 0.05% (1 lb per 100 gallons) to reduce stress and deter parasites.
4. If parameters are off, do a 20-30% water change with dechlorinated, temperature-matched water.

Long-Term

Ensure the pond has sufficient depth and aeration for tench. They are prized fish—unavailable in the US—so extra care is warranted.