Common Pond Fish Diseases: Symptoms, Causes and Treatment

Most pond fish illnesses are linked to water quality and stress, and many are easier to treat when caught early. This guide covers the most common problems in koi and goldfish, how to recognise them and what to do.

Always start with the water

Before treating any disease, test your pond for ammonia, nitrite, nitrate and pH. Poor water lowers fish immunity and is behind most outbreaks. A partial water change with a pond dechlorinator often helps straight away. See our test kits.

Spring is the danger time

As water warms in spring, parasites become active before fish immune systems have recovered from winter. Watch fish closely when water reaches 8–15°C, and avoid treating with some medications below the temperature on the label.

Common diseases

White spot (ich)

Signs: small white spots like grains of salt on the body and fins, flashing (rubbing against surfaces), clamped fins.

Treatment: a broad-spectrum pond treatment such as Tetra Pond MediFin. Treat the whole pond and repeat as directed, because the parasite has a free-swimming stage. Our white spot guide explains the life cycle.

Flukes and other parasites

Signs: flashing, jumping, gasping, excess slime, fish hanging near water inlets.

Treatment: a parasite treatment suitable for ponds. Browse our pond fish treatments. A microscope check by a koi specialist or vet confirms which parasite you're dealing with.

Ulcers

Signs: red patches that become open sores, often in koi in spring.

Treatment: ulcers are bacterial and usually follow parasite damage or poor water. Improve water, treat parasites, and use an antibacterial treatment such as API Pond MelaFix to support healing. Serious ulcers may need a vet and topical treatment.

Fin rot

Signs: ragged, receding fins with white or red edges.

Treatment: water changes first, then an antibacterial treatment. See our fin rot guide.

Fungus

Signs: fluffy white or grey cotton-wool growths, usually on damaged areas.

Treatment: fungus is a secondary infection. Treat with a pond fungus or broad-spectrum treatment and fix the cause.

Dropsy

Signs: a swollen body with scales sticking out like a pine cone.

Treatment: dropsy is a sign of serious internal illness and is hard to treat. Isolate the fish and seek advice quickly.

How to treat a pond safely

• Measure your pond volume accurately: overdosing and underdosing are both common.
• Switch off UV clarifiers during treatment, as UV can break down some medicines.
• Increase aeration with an air pump such as the Evolution Aqua Airtech; many treatments lower oxygen.
• Don't mix treatments unless the label says it's safe.

How to prevent disease

• Quarantine new fish for at least three weeks before adding them to the pond.
• Don't overstock or overfeed. See our pond fish food guide.
• Keep good filtration and clean it in pond water. See pond filters.
• Remove sludge and debris. See pond cleaning.
• Watch your fish every day at feeding time.

New to koi? Read Koi Care for Beginners.