Koi ponds must be deep enough to keep the fish safe from predators and warm in winter, especially in the British climate, and must also have protection for human safety. They need to be situated in the right place and made with suitable materials, properly fitted. The ponds need a filtration system to keep their water fresh enough for the carp.
Where to put the pond
Ponds should be situated away from trees whose growing roots might puncture them and release water, and should not be situated where trees will not deposit considerable number of leaves, especially if they are toxic leaves or seeds. You should also ensure that your fishpond is not too close to your house, as it might threaten the foundations. It should be away from children's play areas and should have security netting.
Depth and width
While some fish owners keep their carp in shallow ponds, this is not safe. In the British climate, especially as you go further north, shallow water can freeze entirely, killing all the pond fish and herons seeking fish prey that appreciate shallow water. Most experts recommend 4 feet 6 inches, about 1.33 metres minimum. 2 metres is considered more appropriate. 10 feet
(3 metres) wide is a reasonable width, though some go to 15 feet.
If you want any pond with vertical sides, you must have a solid construction, e.g. concrete. At this point, professional help will not go amiss. If you are excavating it yourself, do not do so alone, as a cave-in can kill you. Fibreglass sides are not recommended, as fibreglass is toxic to fish.
A butyl liner can be used. This should more than cover the total surface area of pond and sides. If you use liners, ensure that the bottom and sides of the pond are free from stones, especially sharp ones, and the bottom is covered with sand before the liner goes on.
Pond care is essential. Fish requires oxygenated water, or else they suffocate. In the wild, this is provided by streams flowing into ponds. However, artificial ponds require oxygenation. A pond aerator will be necessary, it should be one that can aerate the pond's size. A fish tank aerator is not good enough.
You will need a filtration system, otherwise your fish will be swimming in water polluted by their own excrement. Koi filters must be big enough to deal with the size of your pond, so you need to purchase them from a specialist source. The pump must be of suitable size. It should be wired by a professional electrician, because when electricity is close to water, mistakes are fatal.