Koi

1 Species Overview

Koi, also known as ornamental carp, have the Latin name Cyprinus carpio. In China they have long been kept as both feng shui fish and pet ornamental fish because of their distinctive beauty, and after being introduced to Japan they became equally beloved there. They are also not especially difficult to keep.

Taxonomy: Kingdom — Animalia; Phylum — Chordata; Subphylum — Vertebrata; Class — Actinopterygii; Subclass — Neopterygii; Order — Cypriniformes; Family — Cyprinidae; Genus — Cyprinus; Species — Cyprinus carpio.

2 Morphological Characteristics

Through continuous selective breeding, koi now come in a very large number of varieties, earning them nicknames such as “living jewels of the water” and “swimming works of art.” So just how many colors can a koi display? Here is a brief introduction.

Koi are highly valued ornamental fish and form a very large branch with many different varieties. Their body coloration is brilliant and extremely varied. By color pattern they are broadly divided into single-color, two-color, and three-color types. The most classic is the Red-and-White (Kohaku) koi, with fresh red markings over clean, bright white.

Some koi have very pure, deep, and lustrous body color. For example, among Red-and-White lines, the Dainichi strain shows orange-tinged red markings that look bright and vivid, while the Senshu strain shows deeper, darker red that appears more subdued.

Based on body color, koi are categorized by enthusiasts into Red-and-White, Sanke (Taisho Sanshoku), Showa Sanshoku, Bekko, Asagi/Shusui, Kingyo (metallic), Hikari Moyou, Ginrin (silver scales), Tancho, and more. For a koi, body color is a central part of its appeal, and the quality of that color becomes an important standard for judging a fine specimen.

3 Care & Husbandry

Although koi are not demanding about water quality and are not hard to keep, keeping them healthy and their colors bright still requires attention to proper husbandry. Here is what you should know.

1. Like goldfish, koi are not strict about water temperature or quality and can live in a range of roughly 2–30°C. They adapt strongly to their environment, but they cannot tolerate sudden temperature swings. Long-term captive fish can endure a rise or fall of 2–3°C; a swing beyond that makes them prone to illness, and a 7–8°C jump can leave them lying at the bottom, refusing food and weak — an even larger sudden change can kill them outright. The ideal living temperature is 20–25°C, where fish are active, eat well, stay healthy, and show the brightest color. Keeping the temperature in this comfortable range for steady, rapid growth is ideal. Koi absorb dissolved oxygen through their gills to power their metabolism, so ample oxygen in the water is the key to keeping them well; oxygen demand rises with temperature (about 35 mg per liter per kg of body weight per hour at 5–6°C), proving that warmer water means higher oxygen consumption.

2. For a new tank or pond, change a larger volume of water daily; once the system is matured, twice-weekly changes are enough.

3. Observe the fish every day. If a fish stops eating or hides in a corner, consider that it may be sick.

4. When moving fish, use a soft knotless net to avoid injury.

5. If you see a fish rubbing its body against objects, it likely has parasites — consider anti-parasitic treatment.

6. If the water stays cloudy, check whether the filtration system is failing.

7. When a fish is ill, do not rush to isolate it. If isolation is needed, use a quarantine tank with a complete filtration system.

8. Do not buy large numbers of fish from different farms at the same time, to avoid cross-infection.

9. Keep common medications on hand: coarse salt, Japanese yellow powder (nitrofurazone), potassium permanganate, antibiotics, and anti-parasitic treatments.

10. When buying fish, consider your water volume and filtration capacity — do not stock too densely.

11. How to keep koi color vivid: (a) Keep the water pure — remove nitrite as much as possible, strengthen biological filtration, and if possible drain bottom water and add well-aerated fresh water daily; increase dissolved oxygen and air flow, keep water soft, thoroughly clear iron from the water, and feed slightly more calcium-rich food. (b) For bright red markings — keep pH at 7.0–7.2 by placing coral sand or oyster shell in the filter or return flow, and feed moderately more food containing natural carotene. (c) For deep black markings — keep the temperature relatively low and pH slightly below 7.2, reduce direct sunlight exposure, cut back on fatty feeds (which reduce excretion), and temporarily raise water hardness at a specific period to intensify the black “sumi” before returning to normal hardness.

4 Habitat & Living Environment

Koi are not demanding about water quality, eat a varied diet, and breed readily. They are sturdy, colorful, and can live 60–70 years. What should you watch for in their environment? A newly built concrete pond must have its alkaline residue removed before use: fill it with a 0.25% phosphoric acid solution, soak for one to two days, then rinse the walls clean with fresh water.

5 Feeding Guidelines

Although koi are omnivorous, you must still feed them correctly and not blindly toss in anything. What you feed also affects whether their color fades and whether they stay healthy. Choose feed that looks good, is free of mold, caking, spoilage, or off odors; the best pellets smell of fish and a mild fishy aroma in addition to the base grain scent — much like choosing good rice at the supermarket.