Healthy-looking koi usually swim upright, breathe comfortably, and show intact skin and fins appropriate to their variety. Appearance is a useful baseline, not a diagnostic test. Some infections are not obvious externally, and normal behavior varies with temperature and the individual.
Watch Swimming and Breathing
Look for smooth movement, balanced turns, and the ability to hold position without struggling. Compare breathing at rest with the fish's usual pattern. Persistent surface gasping, rolling, or a sudden change in breathing deserves attention even when color and scales look attractive.
Look at Skin, Scales, Eyes, and Fins
- Compare both sides as the koi turns; note new sores, swelling, or unusual mucus.
- Recognize reduced Doitsu scaling instead of assuming every unscaled area is damage.
- Compare the two eyes for new cloudiness, injury, or asymmetry.
- Look for new tears, erosion, or sustained fin clamping.
Separate Normal Pattern from New Damage
Different varieties have different colors, shine, and scaling. Strong color does not prove health, and slow pattern development is not necessarily disease. Dated photographs are useful when a mark is uncertain. Rapidly changing tissue damage, raised scales, or an open lesion needs investigation.
Use Feeding as One Observation
When feeding is appropriate, watch access to food, swallowing, and changes from normal interest. A friendly fish is not automatically healthy, and a shy fish is not automatically sick. Cold conditions, recent transport, and environmental trouble can all affect appetite.
Combine the View with Records
Check water and equipment, document changes, and consider source and exposure history. For a purchase, use seller observation and questions. For established fish, use the daily health checklist and a structured examination routine.