Ornamental Koi are mostly bred in Japan and shipped to rest of the world. Many years of selective breeding had led to development of various colorful varieties of Koi but all will need the same water requirements. They are relatively hardy fish and can tolerate poor water conditions but not for prolonged periods. If you are serious about Koi keeping, then continual good water quality is must to maintain your Koi at their peak condition. A pond for keeping Koi has more stringent requirements compared to a normal garden pond.
Although most Koi sold are young Koi and averages from 20 cm (yearlings) to about 50 cm, Koi can grow to as large as 1 meter in length. Size and bulk is one criterion when Koi are judged at Koi shows. Koi need space to thrive and some Koi experts have claimed that growth rate is proportional to pond size and volume of water per Koi.
Just like in aquarium fish keeping, a larger volume of water or pond would also provides more stability in the water condition. Often, this is limited by available space for your pond and usually not by cost. I am not sure about other countries, but in Singapore, my pond builder has told me that I can have a slightly larger (or smaller) pond for the same price (like buying a shirt!). If you are planning to keep 9 Koi (each about 70 cm long), the pond dimensions should be at least that of a king-size bed. Believe me, once you are bitten by the Koi-keeping bug, you would not want to stop at 9! Hence, you should go for the largest pond that you can fit into the available space in your garden or house.
Koi needs oxygen and so does the bacteria in the biological filter. Dissolved oxygen level of 8 mg/liter in water temperature of about 25 deg C is almost ideal since it is almost at saturation point. I used a water test kit to measure this. Note that plants (including algae) in the water also use oxygen at night. Plants take in carbon dioxide and produce oxygen only in the day, during photosynthesis. Some form of aeration in your pond design is a must. Oxygen is absorbed by the water when it comes in contact with air. Any way to increase the interface area between the pond water and air would be good. This can be achieved through use of a waterfall, fountain, venturi pipe water return system or air pumps that create streams of bubbles in your ponds.
There is a need to keep the water cool, especially during summer or if you live in hot, sunny country like Singapore where the daily temperature ranges from 25 deg C at night to 34 deg C in the day. Warm water does not carry oxygen as well as cool water. In any case, some shade would be good for the Koi. I have read about Koi getting sun-burnt from prolong exposure to direct sunlight.
Pond depth should ideally be at least 1.2 meter or about 4 feet. Pond surface area is usually limited by the space available in our home or garden. A deeper pond would allow for greater pond volume per unit pond surface area. A deeper pond would also help keeps the water cooler in hot, sunny weather. Hence, a deeper pond is good for the Koi growth but one has to be mindful of the proportion of the depth versus size of the pond.
pH is a measure of the acidic or basic (alkaline) nature of a solution. Normal tap water or fresh water should have pH level of 7. Water with a lower pH level is deemed acidic while alkaline water would have higher pH. Chalk, limestone, coral or seashells dissolved in water will produce higher pH. It is important to maintain the right pH level get more info for the right type of fish. For example, African cichlids which originate from Lake Malawi requires high pH and a rocky environment, while South American tetras and cichlids which originate from the rivers of Amazon require low pH. Low pH or sudden drop in pH level kills Koi and many other fishes. It is generally better to have pond water for Koi at with slightly higher pH (7.4 is ideal. Not more than 8.0). Water test kit can be used to measure pH. Many Koi keepers, myself included, placed coral and seashells in their filtration system to maintain or buffer the water from sudden pH level changes in their Koi ponds.
Ammonia, Nitrite and Nitrate
Fish produces ammonia as a waste product and a good filter system must be provided to remove this ammonia from the water. If a biological filter is used, the bacteria in the filter will convert the ammonia to nitrite and then from nitrite to nitrate. Nitrate is relatively less harmful to fish and Koi than ammonia or nitrite. Ideal levels of ammonia and nitrite should be zero or near zero and this can be achieved using a good biological filter system. Nitrate level should be less than 25 mg/liter. I used a water test kit to measure nitrate and nitrite levels. The only way to reduce nitrate is through partial water changes. Water plants to absorb the nitrate but may not completely remove all nitrates from your pond. Excessive nitrates may lead to algae bloom and turn your water green. Koi needs to eat regularly and eats a lot to maintain their bulk and grow to their fullest potential. This means a lot of waste matter is produced and have to be processed by the filtration system. A rule of thumb is that the size of the filter system should be one-third of your pond size. Specially designed, all-in-one filter system available commercially may be able to achieve the same effect with a reduced size.
Learn how to keep water, then the Koi will keep themselves. Hence, building an adequately sized pond with a good filtration system is the first step towards having good water quality and healthy Koi. Regular water testing and partial water changes are essential too.