The key to keeping healthy happy Koi Carp is water quality, without plenty of clean well oxygenated water you are likely to experience constant health problems including parasite infestation and bacterial disease.

The size of a Koi pond is a question often asked. The large volume of water which is well filtered with a few koi will mean health problems kept to a minimum.A koi pond needs to be 1500 gallons with a minimum depth of at least 3 feet. Koi like stable water parameters which includes temperature. The temperature of a pond will fluctuate to much if it is shallow, a 1500 gallon koi pond would be fully stocked with around 8 koi averaging 30cm in length.

Water temp	What is important with regard to temperature in a koi pond is that it is constant and does not swing rapidly. I would suggest the absolute minimum size for a Koi pond to be 3ft deep and 1500 gallons. Although an average depth of 5 foot would be my recommendation. Generally a larger volume of water will provide a better living environment for its inhabitants. However I must stress that it is a myth that Koi grow to the size of the pond. Koi are large fish and if kept in good conditions will continue to grow. In a shallow 3 ft deep pond the Koi will tend to grow fatter rather than the slender appearance you would associate with a large Koi you may see in a dealers premises.
PH	Ideally between 7.0 to 8.0 although 6.8 to 8.5 is acceptable, the important thing is that the PH is stable and does not alter much. The PH will affect the general health and immune system of Koi. If you find your PH alters, algae and in particular blanket weed growth can often be the cause. If you have a high PH check for run of from cement and rocks with a heavy lime content.
Ammonia	0 - Also note that any Ammonia present will be more toxic with a high PH.
Nitrite	0

The health of Koi falls into three main areas:- parasites, viruses and bacterial disease which includes ulcers and fin rot etc.

GENERAL SYMPTOMS - of a health problem are reduced appetite, flashing - rubbing against objects, "hanging" in particular near water inlets and air stones, listlessness, loss of balance and clamped fins. If you notice any of the above behavior a closer examination is required.

EXTERNAL EXAMINATION - Look for ulcers and fin rot, excess mucus, loss of colour, tumours and malformations, weight loss, swollen eyes, swollen abdomen. Parasites such as White Spot, Costia, Trichodina, Chilodonella and Flukes will require examination of a mucus scrape using a microscope.

The identification of common parasites which affect Koi is important in order to ensure correct treatment. A microcope is necessary as most parasites can not be seen by the naked eye.
See photographs and treatments here.

Fish are susceptible to a variety of infections. Some diseases show no outward signs and do their damage inside the body, attacking vital organs and systems. Luckily, this type of infection is rare, as the first outward signs are normally a number of seemingly healthy fish dying for no apparent reason. The most common infections encountered by owners are usually associated with ulceration and fin-rot.
See photgraphs and treatments here

Koi viruses can destroy complete koi collections almost overnight and can be very difficult to treat.
more about koi viruses