A healthy koi swims calmly, feeds actively, and exhibits controlled behavior in the pond. Changes are often the first sign that something is wrong.

Common symptoms:

• Lethargy or apathetic behavior
• Rubbing against walls, stones, or plants
• Clamped fins
• Whitish coatings or spots
• Redness or open sores
• Rapid breathing or gasping for air
• Loss of appetite
• Uncontrolled swimming
• Staying at the bottom or surface

Those who observe their koi daily can detect many diseases at an early stage.

The most common koi diseases

1. White Spot Disease (Ichthyophthirius)

White spot disease, also known as "Ich," is one of the most common parasitic diseases in koi. It is caused by a parasite that embeds itself under the fish's skin.

Identification features:

• Small white spots on skin and fins
• Frequent rubbing against stones or plants
• Apathetic behavior
• Reduced appetite

Treatment:

• Isolate affected fish if possible
• Controlled increase of water temperature
• Use special anti-parasitic preparations
• Stabilize water quality

2. Fin Rot (Bacterial Infection)

Fin rot is usually caused by bacteria that proliferate due to stress, injuries, or poor water quality.

Identification features:

• Frayed or thinned fins
• Discoloration at the fin edges
• Sluggish swimming movements
• General weakness

Treatment:

• Perform regular water changes
• Improve water parameters
• Use antibacterial agents
• Strengthen the immune system with high-quality food

3. Gill Necrosis (Branchiomyces)

This disease affects the gills of koi and is caused by mold fungi. It occurs more frequently in poor water quality.

Identification features:

• Rapid or shallow breathing
• Gasping for air at the surface
• Pale or discolored gills
• Reduced activity

Treatment:

• Increase oxygen levels
• Significantly improve water quality
• Use antifungal agents
• Act early, as the disease can quickly become life-threatening

4. Carp Pox (Cyprinid Herpesvirus)

Carp pox is a viral disease that becomes visible mainly at low temperatures. The symptoms outwardly resemble waxy skin changes.

Identification features:

• Whitish, waxy growths
• Changes in skin and fins
• Usually normal behavior despite symptoms

Treatment:

• No direct cure possible
• Optimize keeping conditions
• Maintain stable temperature

Carp pox is usually harmless but visually disruptive.

5. Dropsy

Dropsy is a serious disease in which fluid accumulates in the koi's body. It is often caused by bacterial infections or poor water conditions.

Identification features:

• Severely bloated abdomen
• Protruding scales ("pinecone effect")
• Swollen eyes
• Swimming problems and apathy

Treatment:

• Isolate affected animals
• Immediately improve water quality
• Targeted treatment with medication

In many cases, the disease is unfortunately fatal.

6. Fish Tuberculosis

Fish tuberculosis is a chronic bacterial disease caused by Mycobacterium. It is considered difficult to treat and can, in rare cases, be problematic for humans.

Identification features:

• Severe weight loss
• Sunken body parts
• Ulcers or skin changes
• Weakness and emaciation

Treatment:

• Focus on prevention
• Maintain consistently high water quality
• Avoid stress
• Isolate severely affected animals if necessary

7. Koi Herpesvirus (KHV)

The Koi Herpesvirus is one of the most dangerous diseases in koi keeping. The viral infection is highly contagious and can affect entire populations.

Identification features:

• Apathetic behavior
• Loss of appetite
• Skin hemorrhages and ulcers
• Severe gill damage
• Breathing problems

Treatment:

• No direct cure available
• Maintain quarantine
• Always introduce new koi under controlled conditions
• Regular disinfection and control

Prevention is particularly important with KHV.

8. Swim Bladder Disorder (Swim Bladder Defect)

The swim bladder allows koi to maintain their balance in the water. With a disorder, the fish often lose control of their swimming position.

Identification features:

• Tilted or sideways swimming
• Difficulty diving
• Constant floating at the surface
• Uncontrolled movements
• Partially bloated body

Treatment:

• Check water parameters
• Reduce stress
• Use easily digestible quality food
• Maintain stable temperature
• Treat specifically for bacterial causes

If the cause is detected early, many koi can recover.

Prevention is the best protection

The best treatment is always good prevention. Many diseases only arise due to poor water quality, stress, or inadequate care.

Important measures:

• Regular water changes
• Control of all water parameters
• High-quality koi food
• High-performance filter technology
• Good oxygen supply
• Keep new koi in quarantine
• Daily observation of the animals

A stable and professionally maintained pond is the most important foundation for healthy koi.

Conclusion

Koi diseases can have many causes and often develop gradually. Those who observe their animals carefully and react early can often prevent major damage.

Healthy koi need:

• clean water,
• stable water parameters,
• high-quality technology,
• balanced nutrition,
• and as little stress as possible.

Because only under optimal conditions can koi fully develop their beauty, vitality, and impressive charisma.