Koi ponds are peaceful, beautiful, and filled with living jewels… until one morning you walk outside and something looks off. A koi is hiding. Another is rubbing on rocks. One has a weird white patch. Suddenly, your zen paradise feels more like an emergency room.
Don’t panic—every koi keeper eventually deals with diseases or parasites. The good news? Most problems can be stopped quickly if you know what to look for. Consider this your entertaining-yet-essential guide to the most common koi health issues and how to spot them before they turn into a full-blown pond drama.
1. Ich (White Spot Disease)
What it is: A parasitic infection caused by Ichthyophthirius multifiliis. It’s basically the measles of the fish world—but itchier.
How to spot it:
- Small white spots on fins and body (like salt grains).
- Fish rubbing or “flashing” against surfaces.
- Clamped fins or lethargy.
Fun koi fact: Ich spreads rapidly in stressed fish or sudden temperature drops. It’s the parasite version of “kick them while they’re down.”
2. Flukes (Gill and Skin Flukes)
What it is: Tiny worm-like parasites (Dactylogyrus and Gyrodactylus) that love to camp out on koi skin and gills.
How to spot them:
- Rubbing or flashing on pond walls.
- Rapid breathing or gasping.
- Excess mucus or cloudy patches.
- Redness around gills or fins.
Why it matters: Flukes are sneaky—they don’t always show visible signs until the infection is advanced. A microscope is your best friend here.
3. Costia (Ich’s Tiny, Nasty Cousin)
What it is: A microscopic parasite (Ichthyobodo) that thrives in cold water and stressed fish.
How to spot Costia:
- Grey or bluish patches on the body.
- Clamped fins.
- Lethargy and isolation.
- Heavy slime coat production.
Pro tip: If your koi look like they’re covered in dull, dusty film, suspect Costia.
4. Trichodina (The Spinning Saucer Parasite)
What it is: A saucer-shaped parasite that looks like a spinning coin under a microscope.
How to spot it:
- Flashing or rubbing behavior.
- Excessive slime coat.
- General irritation but few external marks.
Fun fact: Trichodina is often linked to poor water quality. Clean ponds rarely have this troublemaker.
5. Anchor Worm
What it is: A visible parasite (Lernaea) that literally anchors itself into the koi’s skin.
How to spot it:
- Worm-like threads protruding from the skin.
- Red, inflamed areas where worms attach.
- General irritation or jumping.
Pro tip: Anchor worms look small, but they pack a punch. They should be removed carefully with proper treatment—not with tweezers!
6. Ulcers (Open Wounds on the Body)
What they are: Open sores often caused by Aeromonas and Pseudomonas bacteria. Usually secondary to stress, parasites, or injuries.
How to spot ulcers:
- Red sores or open wounds.
- Missing scales.
- Redness around the lesion.
Why it matters: Ulcers escalate quickly. Treat the underlying cause (parasites, water quality), not just the wound.
7. Fin Rot
What it is: A bacterial infection that causes fins to fray, split, or deteriorate.
How to spot fin rot:
- Ragged or torn fins.
- Red streaks or inflammation.
- Slow deterioration over days or weeks.
Prevention: Excellent water quality and low-stress environments keep fin rot at bay.
8. Dropsy
What it is: A symptom (not a disease) of organ failure, often caused by severe infection or poor water quality.
How to spot dropsy:
- Scales sticking out like a pinecone.
- Swollen belly.
- Lethargy or inability to swim normally.
Urgency level: High. Dropsy is serious and often fatal without quick intervention.
9. Swim Bladder Issues
What it is: A condition affecting buoyancy, sometimes caused by infection, pressure from internal swelling, or poor diet.
How to spot swim bladder problems:
- Koi floating upside-down or sideways.
- Difficulty staying submerged.
- Erratic swimming.
Fun fact: Not all upside-down koi are dying. Some are just temporarily unbalanced—but it’s always worth checking.
10. Water Quality Stress (The #1 Imposter Syndrome Illness)
What it is: Not a disease, but the root cause of most problems. Poor water quality weakens koi and opens the door to parasites and infections.
How to spot water-quality stress:
- Gasping at the surface.
- Clamped fins.
- Red streaks in fins.
- Lethargy or random darting.
- Cloudy or smelly water.
Golden rule: If your koi look sick, test the water first.
The Early Warning Signs Every Koi Keeper Should Know
If you see any of these behaviors, something is wrong—even if you can’t spot the exact disease yet:
- Not eating (the koi equivalent of “call 911”).
- Hiding or isolating.
- Clamped fins.
- Breathing heavily.
- Rubbing against surfaces.
- Erratic or distressed swimming.
Koi rarely “just act weird.” Behavior is your first clue.
Koi diseases sound scary, but with good observation and fast action, most issues are absolutely treatable. The real secret? Healthy water, low stress, and regular checkups keep diseases from ever gaining a foothold.
Learn the signs, watch your fish, test your water, and act early. When you understand koi health, you transform your pond from a source of worry into the peaceful sanctuary it was meant to be.
And your koi? They’ll reward you with bright colors, calm swimming, and that unmistakable “we’re thriving” energy.