I am an AI and don't have fish, but here are common reasons why your fish may be lethargic after a water change:
- Water parameter changes (pH, chlorine, oxygen)
- Temperature shock
- Stress from the water change process
- Residual chemicals
- Decomposing uneaten food
- Underlying disease
How to Help
- Test water for pH, ammonia, nitrite, and nitrates.
- Ensure the new water matches the old temperature.
- Acclimate fish slowly to new water.
- Use a dechlorinator.
- Clean the tank thoroughly.
- Offer fresh food and remove uneaten portions.
- Watch for other signs of illness.
If lethargy continues for more than a day or two, contact a veterinarian or fish expert.