If your koi appear to be dozing off, swimming lethargically or rolling on their sides, there is a chance they have koi sleeping sickness. This is apparently caused by minute blood parasites and supposed to be particularly common in juvenile fish. Laboratory tests haven't confirmed that the disease actually exists in koi, according to the Australian Department of Fisheries, but there is plenty of anecdotal evidence from koi owners. There is no consensus on treatment, although some owners recommend the use of salt baths or green malachite. If your fish are showing the signs of sleeping sickness, it is best to consult a vet who specialises in koi as soon as possible.
Quarantine the affected fish, if only a few individuals are exhibiting symptoms. Move to a freshwater aquarium or another pond.
Remove any leeches with a net from the original pond and from the fish. Leeches might pass on infections.
Keep the infected fish reasonably warm, at around 20-24 degrees C. If the koi start showing symptoms during or just before the winter, consider taking them inside or installing a pond heater.
Feed all your koi a high-quality diet with supplemental live food such as blood worms. This improves their condition and helps them to either recover from or withstand parasites.