If a koi looks sick, begin with the pond and the pattern of symptoms. One injured fish, several gasping fish, and a whole group suddenly refusing food suggest different possibilities. Observation and water tests are more useful first steps than choosing a medicine from appearance alone.
First Checks
- Check that pumps, aeration, and filters are operating.
- Improve aeration promptly if fish are gasping or breathing unusually hard.
- Test ammonia, nitrite, pH, and temperature; include alkalinity and source-water checks as appropriate.
- Record recent fish additions, treatments, refills, equipment failures, and feeding changes.
- Photograph visible signs without repeatedly catching the fish.
Test before major corrections if you can do so without delaying urgent oxygen support. Several fish affected at once is a reason to inspect the entire system immediately.
When a Hospital Tank Helps
A separate system may make observation or prescribed treatment easier, but an uncycled bucket can make the problem worse. Prepare suitable space, filtration, oxygen, a cover, treated water, and close monitoring. Moving a fish is itself stressful; assess whether transfer offers a real benefit.
Avoid Automatic Salt, Heat, and Medication
There is no universal salt dose or hospital temperature for every ulcer, infection, or breathing problem. Heating can change oxygen needs and disease behavior. Salt interacts with plants, some media, and treatments. Antibiotics or medicated baths should follow a diagnosis and a specific plan, not substitute for one.
Get Help for Significant Signs
Ulcers, ongoing respiratory distress, loss of balance, rapid deterioration, or unexplained deaths warrant a fish veterinarian or qualified fish-health service. Send your water log, photographs, timeline, and system details. Do not release a recovering fish simply because a set number of days has elapsed.
Find support through the American Association of Fish Veterinarians. The koi-health guide explains common symptom groups, and the water-quality guide covers environmental checks.