Koi Carp

Koi need ponds, not tanks. Plan for a minimum of roughly 1,000 litres of water per adult koi and realistically several thousand litres for a small group, with robust biological filtration and strong aeration. The pond should be at least 90 to 120 cm deep to keep temperature and oxygen stable and to protect fish from herons and cats. An aquarium is only ever a temporary quarantine or grow-out space, never a permanent home, and pond netting guards against predators and leaves.

Feed seasonally, which is critical for koi. In warm months offer a high-quality floating koi pellet a few times daily in amounts they clear in minutes, switch to an easily digested wheatgerm food in spring and autumn as water cools, and stop feeding entirely once water drops below about 10 C, because their metabolism slows and undigested food rots in the gut. Never overfeed, as leftover food quickly fouls the pond.

Koi are cool to temperate fish, happiest at 15 to 24 C but tolerating roughly 2 to 30 C across the seasons. Aim for pH 7.0 to 8.5 and moderate hardness. A mature, heavily filtered system must keep ammonia at 0 and nitrite at 0 with nitrate low, supported by the large water volume and high turnover. Dechlorinate all top-up and change water, test regularly, and keep aeration running especially in summer heat when oxygen levels fall.

Koi are social shoaling fish that should be kept in groups and are generally peaceful. They coexist well with goldfish and golden orfe of similar care needs. Introduce new fish slowly and quarantine them first, because crowding stresses the fish and overloads the filter.

Provide open swimming space, good depth, and gentle current from the pond returns. Shade from plants or a pergola, marginal planting, and smooth rocks give a varied, natural environment.