Koi care in India: ponds, viruses and time away

Koi are not a separate species. A 2007 study in Genetics Selection Evolution describes the Japanese ornamental carp, or koi, as a coloured variant of the common carp, Cyprinus carpio, developed in Japan in the early 19th century, as documented by breeders in Niigata Prefecture. The US Geological Survey lists koi alongside leather, mirror and Israeli carp as strains bred in aquaculture or kept as ornamentals.

The common carp itself has a long history with people. The same study says its wild ancestor probably lived in the Caspian and Aral Sea basins before spreading west to Europe and east to Asia, and that the Romans probably domesticated European carp early in the 1st century.

No source we found records when koi first came to India, but they are farmed, sold and kept here, and Indian scientists have tracked their diseases. A 2012 risk assessment in the Indian Journal of Virology tested 254 Indian fish samples for koi herpesvirus, found none positive, and warned that unrestricted koi imports posed a risk to aquaculture.

Since then two serious koi viruses have been confirmed in India. Carp edema virus, or koi sleepy disease, was reported on a Kerala koi farm in June 2015, only in newly introduced fish, with signs that it was widespread. In 2022 the first koi herpesvirus in Indian koi was reported from aquarium shops, rearing tanks and grow-out ponds in Chennai, with heavy losses.

The Merck Veterinary Manual says several pelleted foods are made specifically for koi and goldfish in outdoor ponds, and that the amount should be reduced in colder weather because the fish are less active and need less. It adds that a smaller pack bought often stays fresher than a large one kept for months, which matters in humid Indian weather.

Overfeeding fouls a pond as surely as a tank. Leave each day's portion measured out for a sitter, and do not let visitors throw bread or snacks into the pond.

Meals a day: warm months, regular small meals, as much as is eaten quickly; cold months (north indian winter), fewer, smaller meals as appetite drops.

Cefas, the UK government's fish health laboratory, says koi herpesvirus (KHV) affects common carp, including koi, and causes disease when water is between 16 and 29 °C. It says there is no treatment, and Merck says the death rate can be close to 100%, with surviving fish able to carry the virus. KHV does not affect people.

A 2012 review in the Indian Journal of Virology tested 254 Indian fish samples and found none positive, but warned that unrestricted koi imports were a risk. In 2022 the Journal of Fish Diseases reported the first KHV in koi in India, in aquarium shops, rearing tanks and ponds in Chennai, with ulcers, bleeding, rotting fins, discoloured gills and very heavy losses. A different virus, carp edema virus, also called koi sleepy disease, was found on a Kerala koi farm in 2015, only in newly introduced fish.

For an owner, the lesson is quarantine. Merck calls quarantine of new fish the best way to keep KHV out of an existing pond, and Cefas advises against buying fish of unknown origin. Never add fish just before a trip, and tell the sitter that no fish, plant or water from elsewhere goes in.

Only in a very large tank, and most koi are pond fish. The common carp they come from can reach 122 cm, according to the US Geological Survey, and Merck says the number of fish for the space should be lower in cold-water systems such as ponds. If you keep koi on a terrace or in a society garden, check your society's rules first and make sure the structure can take the weight of water.

In Indian summers, Merck notes that floating plants shade a pond and help keep the water cool. In the monsoon, ask the sitter to check the pond level and overflow after heavy rain.