Symptoms
Flashing (rubbing) koi
Goal: identify the most likely causes and take safe, high-impact steps first.
Check these first (in order)
- Ammonia / nitrite (any reading is a problem)
- pH + KH (instability causes irritation)
- Temperature (affects parasite activity + treatment options)
- Recent changes (new fish, heavy cleaning, big water change)
- Aeration (microbubbles / gas irritation can trigger flashing)
Common causes
- Parasites — Often affects multiple fish; microscope confirms.
- pH/KH instability — Daily swings or low alkalinity can irritate skin/gills.
- Water irritants — Chlorine/chloramine exposure, ammonia/nitrite stress.
- Microbubbles/gas irritation — Over-aeration or diffuser microbubbles can mimic problems.
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).