Koi are quite sensitive creatures, and it's much easier to prevent koi disease than to treat it. A good koi pond filter is the most important thing you can do to keep your fish healthy. Also, preventing stress (for example, by sudden changes in koi pond temperature) will make a big difference to the health of your koi.
But sometimes the worst happens, however well we look after our koi. Spotting disease early gives you your best chance of treating it. One of the first signs of koi disease may be that a sick koi will leave the group, swimming listlessly on its own. Also look out for loss of appetite, and gasping for air.
Here's information on some of the most common koi diseases and illnesses to help you work out whether you have a problem that needs treating. There are three basic kinds of koi illness: bacterial koi disease, fungal koi disease, and koi parasites.
Bacterial koi disease
It's hard to distinguish between different bacterial koi diseases, but as they're all treated with antibiotics, all that matters is that you can identify them as bacterial.
Bacterial koi diseases are most likely to strike when the water is full of organic debris, so good filtration is important for preventing them. Koi are most vulnerable if they're already weakened by stress, or if they have a wound caused by parasites.
Look for listlessness, visible blood vessels, reddened areas of skin, and gasping for air. Sometimes bacterial diseases can also erode the skin, gills, or tail and fins. And when things are really bad, you may also see dropsy (see below).
Koi fungus (cottonwool disease)
Like bacterial diseases, koi fungus is most likely to develop when the water contains too much organic debris which it can grow on.
Koi fungus looks like cotton wool, and can grow anywhere on the koi. Don't confuse it with the small white lumps that you may see if koi experience a sudden drop in temperature – these are harmless.
Like any animal, even healthy koi naturally carry a certain amount of parasites. But in the restricted pond environment, sometimes koi parasites can infest your fish beyond their ability to cope.
Koi which are already stressed or weak are most susceptible to koi parasites. Koi have a mucus coating over their scales which deters parasites and infection, but weakened koi may have damaged mucus.
You're most likely to have problems with koi parasites in the spring, as parasites and eggs lie dormant over the winter.
Most koi parasites live on the outside of your fish (ecto-parasites). Some of these can be seen with the naked eye, others are microscopic.
Protazoan koi parasites
Protazoan parasites are single-celled, invisible to the naked eye. Koi infected by these parasites may show listlessness and loss of appetite, their fins may close, and you may see them rubbing against the side of the pond, flashing their underbelly, in an attempt to scratch the itch that the parasites cause. These koi parasites can all be cured by the same kind of treatment.
This video shows a koi flashing (the image is green because it's being treated in a malachite bath):
These are a trematode worm. Gyrodactylus hook themselves onto the koi to feed. They're tiny, but can just about be seen with the naked eye. Koi flukes will eat right through the rays of the fin and tail, and this exposes the koi to bacterial infections which may kill them. The experience is also very stressful for the koi. Koi fluke infections progress very rapidly, so keep an eye out for signs of these koi parasites when you feed your fish.
Dactylogyrus – 'gill fluke' – attaches itself to the gills of your koi, damaging them. Koi with gill fluke will breathe rapidly at the surface of the pond, gasping for air.
Anchor worm – lernea
These koi parasites are a kind of copepod – a tiny crustacean. They're up to 5mm long, so quite visible. Look for a transparent worm near the rear of the fish. Remove affected koi from the pond for treatment, and take precautions to treat the pond as a whole, because by the time you've seen them, they'll have shed their eggs into the water.
Fish louse – argulus
Another kind of copepod, the fish louse is about the same size as the anchor worm – 5mm – but looks like a lump of greenish-brown jelly with eye spots. You can find it anywhere on your koi. Treat like anchor worms – remove the koi and treat the pond.
This video shows argulus:
Gill louse – ergasilus
These copepods are v-shaped. As the name suggests, they feed on the gills, and so affected koi can be seen gasping for air. Treat like anchor worms – remove the koi and treat the pond.
Other koi illnesses
Koi dropsy is a symptom, rather than a cause – it appears when the fish is so ill that it has organ failure. It can be a result of various problems, such as a nasty bacterial infection caused by dirty water (full of organic debris).
Koi with dropsy are bloated, with bulging eyes, and their scales stick out like a pine-cone. Dropsy starts on part of the fish, and gradually spreads to cover the whole koi, at which point the koi will struggle to breathe and swim erratically.
Because dropsy only appears when the organs are damaged, by the time you see dropsy, the fish is usually already too sick to treat. But if caught early, koi dropsy can sometimes be treated by placing the affected koi into water containing salt – 5g per litre, for several hours.
This video shows some koi with dropsy:
Koi ulcers – hole disease
Koi ulcers (also known as hole disease) are easy to spot. They begin as small white lumps, then become red, and eventually a hole appears in the scales and skin. If you catch koi ulcers early, you may be able to treat them with a salt and malachite green bath. If not, you'll need to get a vet in.
The poor fish in this video has a nasty ulcer. Sadly by the time it was spotted, it was already very sick, and it eventually died.
Hollow-back disease is a condition that occurs when koi experience a lot of change in water temperature, stressing their systems. It causes the back to dystrophy, with the back muscle wasting away, and the back sinking into a hollow. The koi will normally recover if you return it to stable warm water, but you may want to feed it antibiotics to stop it catching bacterial diseases while the immune system is weak.
Swim bladder problems
If you find a koi swimming on its side, or upside down, it's probably got a swim bladder problem. This is a rather complicated subject, as it can be caused by several different things, so rather than try and list them, I'm going to point you to this rather helpful forum page.
Here's a video of a fish which looks like it has a swim bladder problem. (It's not a koi – I just came across it in one of the Kauai Hyatt koi ponds!)