Habitat Needs & Maintenance for Outdoor Koi and Pondfish

Although koi fish may appear to be large goldfish, the two are actually distant cousins, with koi being more closely related to carp.

Koi reach 2-3 feet on average with a median lifespan of 20-30 years, sometimes even longer! Traditionally, koi were bred in Japan for hobbyists to collect and place on exhibition due to their exquisite coloration, patterning and scalation, of which more than 100 varieties are recognized. Keeping ornamental koi has become a worldwide passion, and this hardy species makes a great, calming addition to your outdoor pond. Jumpstart your knowledge on creating and maintaining an ideal pondfish habitat year-round.

Caring for koi

Koi are cold-water fish but thrive in water temperatures between 59-77 degrees Fahrenheit. Because their metabolism reacts to the temperature of the water that they live in, you’ll want to be sure your pond has both shallow areas as well as spaces that are deep enough so it does not freeze to the bottom. Experts suggest that seasonal ponds should be kept at a depth of 2 feet, while year-round habitats are recommended to reach 5 feet. During winter months, when the water in your pond becomes cold, the Koi fish's metabolism slows down drastically; they spend most of their time inactive, treading water at the bottom, where the warmest water is located.

Aside from depth and temperature, there are other important requirements that must be considered when planning to incorporate koi into a pond habitat.

Shade to sun ratio

Koi prefer a pond within a specific temperature range, and water which becomes too hot can cause unnecessary stress.

If your pond is situated in an area where it receives a few hours shade daily, or is partially shaded throughout the day, this will provide an ideal climate. If you live in a particularly hot region, have a small, shallow or densely fish-stocked pond, it is advisable to shade the area with plants, non-toxic water dyes, fish shelters or shade sails to make the fish more comfortable.

The bright colors of koi put them in danger of being preyed on by cats, foxes, raccoons, otters, kingfishers, herons, badgers and other wildlife. Shading a pond in addition to a well thought-out design can help ensure that predators can’t easily see, enter or reach into the pond and harm fish.

Dissolved oxygen levels

As a koi keeper it is imperative that you frequently test your pond water and monitor the dissolved oxygen level. Dissolved oxygen refers to oxygen molecules that are dissolved in the water and available for respiration by aquatic life.

When the level of dissolved oxygen in a pond fall below 4 to 5 milligrams per liter, it can be harmful to pondfish health and has been linked to slow fish growth, persistent illness and parasite as well as bacterial outbreaks. Levels below 2 milligrams per liter are particularly detrimental, and in many cases will lead to the death of your koi. 
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A dissolved oxygen level in the range of 7-9 milligrams per liter of water is ideal for pondfish to thrive.

Warm water does not deliver as much oxygen as cool water, which means koi require more oxygen in higher temperatures (when they are very active and eating more often) and less in cool water (when their activity level is much slower).

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Note that tap water and well water have very low levels of dissolved oxygen. You should pay close attention to the oxygen levels whenever a water change or addition is being performed. Digital meters and manual kits are just some of the equipment available for testing.

Because dissolved oxygen levels in ponds fluctuate daily based on sunlight and water temperature, it is recommended to test the water several times over a period of several days to get the most accurate understanding of the average dissolved oxygen levels in your pond.

Adding a waterfall or fountain is beneficial and can help to replenish the oxygen in a contained pond. However, for ponds with a depth greater than 4 feet, a supplemental aeration system is recommended. In the winter, if the water is being aerated, you will need to create a small hole or outlet for the bubbles to escape at the surface of your partially frozen pond.

Feeding and nutrition

Koi can be trained to take food from your hand!

Pondfish should be fed up to 3 times per day for about 5 minutes per feeding so long as they are hungry and there is no excess food left floating, which will deteriorate the quality of the water, decrease dissolved oxygen levels and stress your fish.

In cooler water (under 70 but above 64 degrees Fahrenheit) they should only be fed once per day. When water temperatures drop under 55 degrees Fahrenheit, feeding should stop altogether as their digestive systems slow nearly to a halt. Their appetites do not come back until the water becomes warmer.

In much warmer water (76-82 degrees Fahrenheit), when the fish are very active, be careful not to overfeed, as too much waste can create an unhealthy amount of bacteria.

Before buying koi, contact your veterinarian for suggestions about maintaining a clean, stress-free koi environment, and consult a professional before building a pond to ensure it will become a suitable living space for pondfish.