Koi Disease Identification and Treatment: Ulcers, Parasites and Bacterial Infections

Healthy koi are active, hungry, and display vibrant colour. When something goes wrong, catching it early makes the difference between a simple treatment and a serious loss. This koi disease treatment guide compiles the most common ailments we encounter at Gensou Aquascaping, 5 Everton Park, Singapore, along with the proven remedies our team has refined through over 20 years of pond keeping in the tropics.

Recognising Early Warning Signs

Behavioural changes almost always appear before visible symptoms. Watch for flashing (rubbing against surfaces), isolation from the group, clamped fins, gasping at the surface, or sudden loss of appetite. Red streaks on fins, excess mucus, or cloudy eyes are physical indicators that warrant immediate investigation. Make it a habit to observe your koi closely during every feeding session. In Singapore’s warm ponds, pathogens multiply rapidly, so a 24-hour delay in treatment can allow an infection to spread through the entire stock.

Parasitic Infections

Parasites are the most common koi health issue in tropical ponds. Ichthyophthirius multifiliis (white spot) presents as tiny white cysts on the skin and fins. Treat with malachite green and formalin at the manufacturer’s recommended dose, raising the treatment duration to a full 14 days because the parasite’s life cycle accelerates in warm water. Flukes (Dactylogyrus on gills, Gyrodactylus on skin) cause flashing and excess mucus production. Praziquantel at 2.5 mg per litre is the standard treatment, repeated after seven days. Anchor worms (Lernaea) and fish lice (Argulus) are visible to the naked eye and can be removed with tweezers before treating the pond with an organophosphate or diflubenzuron.

Bacterial Ulcers

Ulcer disease, caused primarily by Aeromonas and Pseudomonas bacteria, is arguably the most destructive condition in koi ponds. It begins as a small red sore and can rapidly excavate deep into muscle tissue. Mild cases respond to topical treatment: net the fish, gently clean the wound with dilute povidone-iodine, apply an antibiotic wound sealant like Tricide-Neo paste, and return the fish to clean water. Severe or multiple ulcers require injectable antibiotics administered by a veterinarian experienced in fish medicine. Maintaining water quality is the single most effective preventative measure against bacterial infections.

Dropsy and Internal Infections

Dropsy presents as a swollen abdomen with scales protruding outward, giving a pinecone appearance. This indicates severe internal organ failure, often from bacterial infection of the kidneys. Unfortunately, by the time visible dropsy appears, the prognosis is poor. Isolate the affected fish immediately and attempt treatment with medicated food containing antibiotics such as kanamycin. Salt baths at 5 grams per litre can draw out some fluid. Prevention through clean water, proper nutrition, and low-stress handling is far more effective than treating established dropsy.

Fungal Infections

Fungal infections appear as white, cotton-like growths on the skin, fins, or gills. They typically occur secondary to a wound or weakened immune system rather than as a primary pathogen. Treat with methylene blue dips at 2 ppm for 30 minutes or add malachite green to the pond at a half dose for three consecutive days. Address the underlying cause as well, whether that is rough handling, parasite damage, or poor water conditions. Once the primary issue is resolved, fungal infections rarely recur.

KHV and Other Viral Diseases

Koi herpesvirus (KHV) is a serious concern for any keeper importing fish. Symptoms include gill necrosis, sunken eyes, and rapid mortality, typically at water temperatures between 18-28 degrees Celsius. There is no cure, and infected fish become lifelong carriers. Singapore’s warm pond temperatures occasionally exceed the virus’s active range, but newly imported fish may carry the pathogen from cooler holding facilities. This is why a strict quarantine period of at least three weeks at 23-25 degrees Celsius is essential for all new arrivals.

Building a Koi First Aid Kit

Every serious pond keeper should have the following supplies ready: salt (non-iodised, for salt baths at 3-5 grams per litre), povidone-iodine (wound cleaning), malachite green and formalin (parasites and fungus), praziquantel (flukes), a set of soft mesh nets, a viewing bowl, waterproof gloves, and cotton swabs. Keep medications stored in a cool, dark place and check expiry dates annually. In Singapore, koi medications are available from specialist pond shops in the Thomson area and through online retailers on Shopee and Lazada, typically costing $15-40 SGD per treatment course.

Related Reading

• Swim Bladder Disease in Fish: Causes and Treatment
• Freshwater Velvet Disease: Piscinoodinium Treatment and Prevention
• Hole in the Head Disease: HITH and HLLE Treatment in Cichlids and Marine Fish
• Marine Velvet Disease Treatment: Amyloodinium in Reef Fish
• Swim Bladder Disease in Aquarium Fish: Diagnosis and Treatment