First check: rule out water stress
Many "disease" behaviors begin as water-quality stress. Before adding medication, test ammonia, nitrite, nitrate, pH, KH, temperature, chlorine/chloramine, and dissolved oxygen if you have a meter. If multiple koi show symptoms at the same time, suspect water or oxygen first.
| Symptom pattern | Common non-disease cause | Immediate first step |
|---|---|---|
| Several koi gasp at the surface or crowd the waterfall | Low oxygen, high nitrite, gill irritation, warm water, medication stress | Add aeration immediately, stop feeding, test oxygen/nitrite/ammonia, and check pumps. |
| Whole pond flashes after a water change | Chlorine/chloramine, pH swing, temperature mismatch | Verify dechlorinator dose, test pH and temperature, and increase aeration. |
| One koi isolates with redness or an ulcer | Localized injury, bacterial infection, parasite damage | Move to clean quarantine if possible and seek qualified fish-health help. |
| Fish are quiet after filter cleaning | Biofilter disruption causing ammonia or nitrite | Test daily, reduce feeding, protect remaining bio-media, and water-change carefully. |
Flashing or rubbing
Often linked to irritation from parasites or water quality. First check ammonia, nitrite, pH, temperature, and recent chemical changes. If water numbers are normal and flashing persists, microscope examination of a skin scrape is more reliable than guessing.
Gasping or hanging near waterfalls
Can indicate low oxygen, gill irritation, parasites, medication stress, or poor water. Increase aeration immediately while checking water. Koi oxygen demand rises after feeding, during warm weather, and when medications or algae die-offs reduce dissolved oxygen.
White spots
Salt-like spots may suggest Ich, but visual confirmation matters. Treatment timing can depend on water temperature and the parasite life cycle.
Ulcers, redness, or open wounds
Often need cleaner water, isolation, and professional assessment. Bacterial problems can progress quickly and may require targeted care.
Clamped fins, isolation, lethargy
General stress signs. Compare behavior across the whole pond and do not medicate blindly if all fish changed after a water event.
If fish are dying, have ulcers, show severe breathing distress, or fail to respond after water correction, contact an aquatic veterinarian or experienced fish-health professional. This website is educational and cannot diagnose individual fish.