Koi Diseases
Diseases of Farmed Koi
There are several common diseases of  koi (Cyprinus carpio var.) which can be recognised and treated successfully by the owner with occasional veterinary assistance. These notes also apply to the same diseases in other ornamental fish species.

Gill diseases
Koi with parasites, bacteria or blockages of the gills will distend their gill covers out wider and breathe more rapidly than normal to obtain oxygen, before dying. Low levels of dissolved oxygen in the water, in contrast, usually cause the koi to sip repeatedly at the surface.

 Costia (10-15 micron metre)
Careful examination of a few sick or very recently dead koi will help identify the cause. If well-defined segments of the normally red gills are white or off-white and slimy, the most likely cause is Bacterial Gill Disease.
This is due to an overgrowth of common water bacteria and flexibacteria, which usually occurs following cold snaps or temperature stress, and in crowded unhygienic conditions. These stressful conditions should be remedied, then the koi can be treated using medication available.
Should the gills appear more normal but perhaps slimy or blotchy, microscopic parasites are the most likely case. These include the tiny protozoans Costia, Chilodonella and Trichodina, which can only be seen under a good microscope.

Gill Flukes 
Gill flukes are highly active 0.5 - 1.5mm flatworms, just visible with a hand lens if affected gill slime is scraped on to a piece of glass and examined carefully in good light. The flukes are best treated with a gill wash mixture . This will help reduce the number of protozoan parasites, but if protozoans are the main problem then a formalin-malachite green mixture is better.
Do not use the two treatments within three days of each other, and maintain good aeration during and after treatment. One or two treatments three to five days apart is usually sufficient, with a follow-up treatment after two weeks in the case of flukes. These treatments are simply added to the pond or tank system at the dose rate on the bottle label and allowed to dissipate.

 Whitespot
 "Whitespot" or "Ich" disease is due to another protozoan, Ichthyophthirius multifiliis, which forms visible white spots or bumps in the skin and gills. The formalin-malachite green treatment will kill "Ich" but only after the spots hatch out, so treatment must be repeated twice-weekly until no white spots have been seen for at least a week.

Skin diseases
Two crustacean parasites are common and problematic, the anchor "worm" Lernaea and the fish "louse" Argulus.

Lernaea
Adult female Lernaea look like a piece of heavy nylon fishing line, about 1 cm long, protruding from the skin, sometimes with two egg sacs visible on the end.
Treatment kills only the free-swimming larvae released from the eggs, so must be repeated weekly until all the adults have disappeared off the fish and for three weeks thereafter.
Paradex is the best chemical to use. It is simply added to the tanks or ponds at the dose rate on the label and allowed to dissipate.
New koi should be quarantined for at least three weeks at 25 degrees Celsius and watched for emergence of the adults. Quarantine of new rocks, plants and water for one week in the absence of koi will ensure the death of any larvae.

 Argulus 
Argulus is a two to four millimetre flat louse-like crustacean, which scurries over the skin and into hiding when koi are handled for examination. Two treatments with paradex, a fortnight apart, will remove most or all Argulus.
New koi should be quarantined for at least two weeks before introduction. The only sure way to keep Argulus out of the farm is to only bring in fish, rocks, plants and water from disease-free sources, as the eggs can survive for up to two months.
Another skin disease that can only be avoided by using disease-free sources is  Ulcer Disease. This is due to a bacterium, Aeromonas salmonicida, and is well known as whitish raised patches on the skin which progress to ugly, ragged, red ulcers.
Affected koi should be removed and destroyed as there is no effective treatment, but never sold or released to ponds or rivers as the disease is dangerous for many fish including trout.

 Koi also sometimes develop coloured lumps on the skin. These are usually relatively harmless tumours known as fibromas, but are unsightly. They can be removed with a sterile scalpel blade if the koi is anaesthetised with masuizai and placed on a soft wet surface. Carp "pox" is a virus infection characterised by soft white lumps on the skin which reappear if excised, but is less common.
Fluke infestations of the skin, as well as the gills, are common. Heavily infested koi appear pale and slimy with red spots or patches in the skin, and may scratch against rocks or tank walls ("flashing"). Treatment is as for gill flukes.

 Fin and tail rot are well known to koi, and similar to Bacterial Gill Disease; the treatment is also the same (see above).
The protozoan parasites affecting gills will also be found on the skin if a scraping of skin slime is examined under a good microscope; again the koi may show "flashing" behaviour.

Other Diseases
Other diseases, less common and usually harder to treat, will occur from time to time. Veterinary advice should be obtained where the disease cannot be solved using the above methods. These include "dropsy" where the abdomen fills with fluid, often due to bacterial infection.
Another cause of "dropsy" which is often overlooked is Kidney Bloater Disease, due to a protozoan called Hoferellus (Mitraspora) infecting the kidney.

Prevention of Diseases
The first step is to quarantine new koi and screen them for the diseases described above, before introducing them to the main pond.  Try and remove parasites by treatment during quarantine. A quarantine period of at least three weeks is advisable.
Good husbandry is then vital in maintaining the koi free of diseases. Avoid high stocking rates and unhygienic conditions due to fouling with food, faeces, dead koi and algae. Reduce temperature shocks and fluctuations. Inspect koi regularly and treat for parasites before they get out of hand.