You can learn a lot about a koi just by watching it—its posture, its movement, its appetite, even the sparkle of its scales. Healthy koi practically radiate confidence; unhealthy koi tend to look… well, like they partied too hard last night. If you’re just starting your koi-keeping journey, this guide will help you quickly spot what’s normal, what’s not, and what makes a koi look like it belongs on the cover of “Healthy Fish Monthly.”
Let’s dive into the wonderful world of koi health—and how to tell if your fish is thriving or just faking it.
1. Bright, Vibrant Colors (The Living Jewel Test)
A healthy koi looks like someone polished it before releasing it into the pond. Their colors—whether red, white, yellow, black, blue, or orange—should be rich, crisp, and evenly distributed. The pigments shouldn’t look dull, dusty, or faded.
- Healthy reds: deep and saturated, never blotchy.
- Healthy whites: bright and milky, not yellowish.
- Healthy blacks: sharp, bold, and inky.
Pro tip: Color can soften temporarily during stress, cold water, or right after a move. But long-term dullness? That’s a red flag—literally.
2. Clear, Shining Eyes
Koi eyes should look clear, bright, and symmetrical. They shouldn’t bulge (unless it’s natural for the variety), sink, or look cloudy.
Healthy eyes: reflect light and track movement. Unhealthy eyes: cloudy, swollen, popped out, sunken, or discolored.
Rule of thumb: If the eyes look like frosted glass, something’s wrong.
3. Smooth, Firm, Unblemished Skin
Healthy koi skin is like silk. It shouldn’t look rough, slimy, patchy, or irritated. The slime coat—the invisible armor that protects koi—should be present but not excessive.
- A healthy slime coat: thin, even, protective.
- Too little slime: fish looks dull, matte, or prone to scrapes.
- Too much slime: indicates stress, parasites, or irritation.
Koi should not have visible wounds, ulcers, raised scales, cotton-like growths, or “pepper” specks (which could indicate parasites like Ich).
4. Perfect Posture (The Swim-Style Check)
A healthy koi swims like royalty—smooth, gliding, and balanced. No twitching, no rapid darting, no wobbling, no floating oddly.
Signs of good health:
- Strong, steady swimming.
- Fins spread naturally.
- Head and tail aligned—no tilting.
- Calm behavior when not feeding.
Unhealthy koi may:
- Cling to the surface.
- Sink or sit on the bottom.
- List sideways or swim in circles.
- Flash (rub against surfaces).
Fun fact: A koi that swims like it’s drunk is not having a good time.
5. Happy Fins (Your Koi’s Body Language)
Koi communicate more with their fins than you might think. Healthy fins are fully open, symmetrical, and free from tears, fraying, or blood streaks.
- Healthy fins: erect, open, and vibrant.
- Clamped fins: closed tightly against the body—an early warning sign of stress or illness.
- Red streaks or tears: potential infection or poor water quality.
Pro tip: Fins are the first part of the koi to show stress. If something looks off, check the water immediately.
6. Great Appetite (The Dinner Bell Test)
Healthy koi LOVE to eat. They should rise eagerly during feeding, jostling with their friends, ready to vacuum pellets like little aquatic shop vacs.
Healthy behavior:
- Eating enthusiastically.
- Competing confidently for food.
- Coming to the surface when you approach.
Not healthy: refusing food, eating slowly, isolating during feeding, or hiding.
Important: Feeding naturally slows when water temperature drops below 55°F (12°C)—that’s normal metabolic behavior, not a health issue.
7. Social, Curious Behavior
Koi are surprisingly interactive. A healthy koi is not shy—it will approach you, swim with the group, and explore the environment.
Healthy koi:
- Swim confidently with others.
- Respond to your presence.
- Show curiosity (yes, koi are nosy).
Unhealthy koi: isolate themselves, hover in corners, hide, or appear lethargic.
Koi are herd creatures—loners are usually stressed or ill.
8. Smooth, Tight Scales
Healthy koi scales lie flat, creating that sleek armored appearance. They should appear:
- Evenly aligned.
- Smooth and reflective.
- Free of bumps, swelling, or gaps.
Pineconing (scales sticking outward) is a serious sign—often indicating dropsy, infection, or organ issues.
9. Steady Breathing
You can tell a lot from a koi’s gill movement. Healthy koi breathe calmly and evenly.
Healthy breathing: slow, steady gill movement while swimming normally.
Concerning signs:
- Rapid breathing.
- Gasping at the surface.
- Hanging near waterfalls for extra oxygen.
- One gill stuck open or closed.
Breathing issues often point to poor water quality or parasites—both deserve immediate attention.
A healthy koi is vibrant, smooth, balanced, active, curious, and always ready for snacks. When your koi looks like a swimming work of art—scales shining, fins spread, colors glowing—you know you’re doing things right.
A healthy koi is bright-eyed, energetic, evenly colored, eager to eat, and perfectly balanced. Anything else is your cue to check water quality, temperature, and overall pond conditions.
Because when your koi thrive, your pond becomes more than water—it becomes a living masterpiece.