Symptoms
Lethargic koi / not eating
Goal: identify the most likely causes and take safe, high-impact steps first.
Check these first (in order)
- Temperature (context for appetite)
- Ammonia / nitrite (fix immediately if present)
- pH + KH (stability; avoid swings)
- Observe breathing rate (fast breathing suggests gill stress)
- Recent feeding/stock changes (spikes or new fish)
Common causes
- Temperature-driven slowdown — Cold water reduces appetite and activity.
- Water quality stress — Ammonia/nitrite or unstable pH/KH suppresses feeding.
- Gill irritation — Parasites or irritation reduces oxygen uptake.
- Overfeeding / waste load — Can trigger ammonia/nitrite spikes.
Safe next steps
- If ammonia/nitrite: do a water change, boost aeration, and stabilize.
- Stabilize KH/pH and avoid large daily swings.
- If symptoms persist and water is stable: consider a parasite plan (microscope is best).