Koi are remarkably resilient fish, but when disease strikes, it can spread through your pond quickly. The difference between a minor health issue and a catastrophic loss often comes down to how early you spot the warning signs.

After more than 20 years treating koi across Gauteng, we have seen every condition in the book. Here are the seven most important signs that something is wrong with your fish — and what you should do about each one.

1. Flashing and Rubbing Against Surfaces

If your koi are scraping their bodies against pond walls, rocks, or the bottom, this is called “flashing.” It almost always indicates a parasitic infection. Common culprits include Costia, Trichodina, and White Spot (Ichthyophthirius). A single flash now and then is normal, but repeated flashing — especially by multiple fish — demands immediate attention.

What to do: Book a microscopic scraping as soon as possible. Visual diagnosis alone is unreliable because different parasites require different treatments.

2. Loss of Appetite

Healthy koi are enthusiastic feeders. When one or more fish stop eating, or approach food and then turn away, it is a red flag. Loss of appetite can indicate gill parasites, bacterial infections, poor water quality, or internal issues.

What to do: Test your water parameters immediately — ammonia, nitrite, pH, and temperature. If water quality checks out, the issue is likely biological and requires professional diagnosis.

3. Clamped Fins and Lethargy

Koi that hold their fins tight against their body rather than fanning them out are showing stress or discomfort. Combined with lethargy — sitting at the bottom of the pond or hovering in one spot — this is a clear sign of illness.

What to do: Isolate the affected fish if possible and contact a koi health specialist. Clamped fins combined with other symptoms can indicate anything from parasites to bacterial infections.

4. White Spots, Patches, or Mucus

Visible white spots on the body or fins typically indicate White Spot disease (Ich). A white, cotton-like film can point to fungal infection. Excessive mucus production — making the fish look slimy or grey — often indicates a parasitic response.

What to do: Do not treat blindly with over-the-counter remedies. Many pond store treatments are too broad and can do more harm than good. A proper scraping identifies the exact pathogen.

5. Red Streaks, Sores, or Ulcers

Red veining in the fins, red patches on the body, or open sores and ulcers typically indicate a bacterial infection, often Aeromonas or Pseudomonas. These bacteria are always present in pond water but attack fish whose immune systems are compromised by parasites, poor water quality, or stress.

What to do: Bacterial infections require prompt treatment. Left untreated, ulcers can become life-threatening within days. Professional assessment will determine whether topical treatment, medicated food, or injection is needed.

6. Gasping at the Surface

Fish repeatedly coming to the surface to gulp air may be experiencing oxygen deprivation or gill damage from parasites. Gill flukes (Dactylogyrus) are a common cause — they damage the gill tissue, reducing the fish’s ability to extract oxygen from the water.

What to do: Increase aeration immediately with an air pump or water feature. Then have the gills examined under a microscope to check for flukes or other parasites.

7. Abnormal Swimming Patterns

Swimming sideways, spiralling, floating upside down, or swimming erratically can indicate swim bladder problems, neurological issues, or severe parasitic infection. In some cases, it may be related to rapid temperature changes or toxic water conditions.

What to do: Check water parameters and temperature stability first. If these are normal, seek professional help immediately — neurological symptoms can deteriorate quickly.

The Bottom Line

The single most important thing any koi keeper can do is observe their fish daily. You know your fish better than anyone — if something looks off, trust your instincts and act quickly.

At KoiDoc, every consultation begins with a comprehensive water test and microscopic analysis. We come to your pond, identify the problem on-site, and provide a targeted treatment plan. No guesswork, no unnecessary chemicals — just science-based care for your koi.

Worried about your fish? Contact us for a free initial consultation and let us put your mind at ease.