Koi Fish

Koi are ornamental domesticated carp that have been refined by Japanese breeders over centuries into living works of art. The most valuable individual specimens have sold for millions of dollars, and koi keeping is a serious competitive hobby — nishikigoi (brocaded carp) culture — with its own classification system, judging criteria, and international shows. For garden pond keepers, koi bring color, scale, and personality to outdoor water features.

Taxonomy & Classification

Koi were long classified as Cyprinus carpio (common carp), but genetic analysis led to the recognition of C. rubrofuscus as the East Asian wild ancestor of domesticated koi. All ornamental koi are the product of centuries of selective breeding from this ancestor in China and subsequently Japan from the 1820s onward.

Color Varieties (Varieties of Nishikigoi)

The major variety groups recognized in competitive koi keeping include:

• Kohaku — white body with red (hi) pattern; the most traditional and highest-regarded variety
• Taisho Sanshoku (Sanke) — white with red and black markings
• Showa Sanshoku (Showa) — black base with red and white markings
• Utsuri — black with white, red, or yellow secondary color in a two-tone pattern
• Bekko — white, red, or yellow base with black tortoiseshell markings
• Asagi — blue-grey reticulated scale pattern with red on abdomen and fins
• Ogon — single metallic color (gold, platinum, orange)
• Butterfly Koi — long, flowing fin extensions developed through hybridization with Indonesian longfin carp

Water Requirements

Pond Requirements

Koi require large, deep ponds: minimum 1,000 gallons for a small group of koi (allow 250–500 gallons per fish). Depth of at least 3–4 feet provides thermal buffering and escape from predators (herons). Essential equipment: oversized biological filter (koi are extremely messy), bottom drain for sediment removal, aeration/waterfall for oxygen and surface agitation, and a UV clarifier to control green water algae. Koi will eat virtually all soft aquatic plants — choose hardy marginals like iris, rushes, and water lotus in separate planting zones or protected pots.

Diet & Seasonal Feeding

Koi are omnivores that graze constantly in warm months. Feed a high-quality koi pellet sized to the fish (small, medium, large) as the primary food; supplement with watermelon, orange, lettuce, and earthworms as treats. Crucially: adjust feeding by water temperature:

• Above 60°F: Standard protein-rich food, 2–3 times daily
• 50–60°F: Wheat-germ-based food (more easily digestible at low temperatures); once daily
• Below 50°F: Stop feeding entirely — the koi's digestive system slows to the point where uneaten food rots in the gut

Common Health Issues

• Koi Herpesvirus (KHV) — highly contagious viral disease with near 100% mortality; no treatment; biosecurity and quarantine of new fish are the only prevention
• Koi Carp Pox — waxy, pinkish lumps on body (viral); unsightly but rarely fatal; no treatment; often clears in warm weather
• Ulcer disease — bacterial infection (Aeromonas); causes open sores; treat with topical antiseptic and injectable/oral antibiotics in severe cases
• Anchor worm (Lernaea) — visible parasitic crustaceans attached to body; remove manually and treat with potassium permanganate or diflubenzuron
• Fish lice (Argulus) — flat, disc-shaped parasites; treat with organophosphate or diflubenzuron
• Dropsy — pinecone-scale appearance from fluid retention; bacterial; often fatal; treat aggressively with antibiotics early