Koi

The Koi, scientifically known as Cyprinus rubrofuscus, belongs to the Goldfish & Koi family group. Adults reach a maximum size of approximately 80 cm and have a fusiform body shape with a fast growth rate. Classified as peaceful, this species primarily occupies the surface, mid-level, and bottom zones of the aquarium and produces a extreme level of biological waste.

To thrive, the Koi requires a freshwater only aquarium with a minimum volume of 3785 litres and a tank length of at least 300 cm. Water temperature should be maintained between 4°C and 28°C, with a pH range of 6.5–8.5 and a general hardness of 4–15 dGH. A moderate water flow is preferred.

The Koi is a rewarding species to keep, provided its core environmental needs are met. This species is available in 3 varieties: Standard Adult, Juvenile (Grow-out), and Butterfly Koi (Long Finned). Each may differ slightly in size, temperament, or care needs, so it is worth checking the specific requirements of the variety you plan to keep.

When setting up an aquarium for the Koi, a moderate water flow best suits their natural preferences. Because this species occupies the surface, mid-level, and bottom zones, the aquarium layout should prioritise adequate surface area with floating cover to make them feel secure near the waterline, open swimming space in the centre of the tank for free movement, and sufficient floor space with hiding spots among rocks, wood, or dense planting. Useful additions include a sand substrate to accommodate their sifting behaviour and smooth gravel or rounded substrate to protect their sensitive barbels. Keepers should note that a tight-fitting lid is essential, as this species is a known jumper, expect substrate disturbance, so lightweight décor should be secured, and live planted layouts may be difficult to maintain, as this species tends to uproot or consume vegetation.

Classified as peaceful, the Koi is a social species that requires the company of its own kind to thrive. When choosing tankmates, shy or slow-feeding tankmates may be outcompeted at feeding time and their energetic swimming style may stress more sedentary or timid species. Selecting tankmates that favour different water layers from the surface, mid-level, and bottom zones can help reduce competition and territorial overlap.

Koi is best for a fishkeeper who wants a species that can settle into a calmer community setup and can provide a keeper who can commit to a proper group of at least 3. It makes the most sense for a hobbyist who will actually build around its needs, especially sand.

The most common mistake is treating Koi like any other community fish and skipping the soft sand it needs to forage safely. Another frequent problem is underestimating its social needs. A token pair or trio often behaves very differently from a real group.

The Koi will eat fry, so breeding efforts in the same tank are unlikely to succeed and is an aggressive eater that may prevent timid tankmates from getting enough food, so target feeding or simultaneous feeding at opposite ends of the tank can help.