Koi

Koi belong in an outdoor pond, not an aquarium — plan for at least 1,000 gallons for even a small group, with a depth of 3 feet or more to protect them from predators and temperature extremes and to give them a stable refuge in winter. Strong pond filtration paired with a UV clarifier keeps water clear and biologically healthy at this scale. Adequate surface agitation from a waterfall, fountain, or dedicated aerator is essential for oxygen exchange, especially in summer heat. Predator netting protects koi from herons and raccoons, which are a real and common threat to outdoor ponds.

Koi are omnivores and do well on a high-quality floating koi pellet, with protein content that should be adjusted seasonally — lower protein in cooler spring and fall water, higher protein in summer. Feeding should stop entirely once water temperature drops below about 50 degrees F, since their metabolism slows dramatically and undigested food can cause serious internal problems. Occasional treats like shelled peas, watermelon, and orange slices are enjoyed in moderation. Feed only what is consumed within about five minutes to avoid foulling the water.

Koi are remarkably interactive for fish, learning to associate their keeper with food and often gathering at the pond's edge or even eating directly from an open hand. They benefit from a mix of varied depths, plantings, and rock formations to explore alongside open swimming areas, and they display genuine social schooling behavior in groups. Their activity follows the seasons closely — expect a marked slowdown in feeding and movement once the water cools in autumn, followed by a return to activity in spring, which is entirely normal rather than a cause for concern.

Parasites such as anchor worms and fish lice are common in outdoor pond systems and should be checked for regularly. Bacterial infections and ulcers typically follow periods of poor water quality or physical injury. Koi herpesvirus (KHV) is a serious, highly contagious disease and a real biosecurity risk — always quarantine any new koi for 2 to 4 weeks in a separate system before introducing them to an established pond. Ammonia and nitrite spikes are especially likely right after winter dormancy, when beneficial bacteria colonies have died back, so test water regularly through the spring restart.