Koi Fish Care Sheet

Koi Fish Species Overview

Koi is the common name for the ornamental varieties of a species of carp. This care sheet outlines basic care needs for a variety of koi species, including: 

• Butterfly koi 

• Doitsu koi 

• Kohaku koi 

• Kujaku koi 

• Matsuba koi 

• Ogon koi 

• Sanke koi 

• Showa koi 

Koi are peaceful, colorful fish known for their large size and impressive beauty.  

Koi have been kept and bred in Japan since the 19th century. In Japanese culture, koi symbolize luck, affection, and love. They were the first fish species that was successfully bred in captivity on a large scale. 

Caring for koi is a rewarding and lifelong commitment—with proper care, these fish can live up to 50 years! 

Depending on their markings, colors, and size, some koi can be very valuable, fetching thousands of dollars at sale! 

Koi Fish Characteristics 

Koi Fish Supplies

To keep a koi happy and healthy, pet parents should have these basic supplies on hand: 

• Appropriate food (dry and frozen) 

• Rocks and plants for decor 

• Pond underlay 

• Pond liner 

• Pond heater/de-icer 

• Water conditioner 

• Pond filter 

• UV clarifier 

• Water test kit 

• Net 

• Thermometer 

• Freshwater pond substrate 

• Air pump 

• Pond skimmer 

Koi Fish Habitat

How To Build a Koi Pond 

Koi ponds can be built from various materials. For optimal success in keeping koi healthy and long-lived, consult a professional builder for help with constructing your pond. During the building process, new koi parents should keep a few things in mind: 

• It’s best to start with an idea of how many fish will be kept in the pond to ensure the habitat will be large enough once completed. 

• Generally, outdoor enclosures are healthier and more stable for koi than indoor aquariums. 

• Koi ponds should be built in shaded areas, outside of direct sunlight. 

• Ponds must be secured against potential predators and curious animals, such as cats, foxes, and coyotes. 

Koi Fish Pond Size

At a minimum, juvenile koi under 6 inches in length can be kept in a 29-gallon enclosure or pond. However, these fish grow quickly and get quite large, so pet parents will need to upgrade the size of their koi’s habitat as they grow. 

As a rule of thumb, adult koi need at least 10 gallons of space for every inch of their body length, so adults that grow to be 24–25 inches in length need approximately 250 gallons per individual. This means that a 10-inch-long fish should be kept in a 100-gallon pond or larger, and a small school of koi may need 1,000 or more gallons of water.

The pond should be at least 3 to 6 feet deep, depending on the species of koi. Water depth is especially important in colder climates that experience freezing, as water can freeze up to 2 feet deep in an outdoor pond during winter.  

Koi Fish Tank Mates 

Koi are peaceful, social fish that thrive when kept in schools. Ideally, you should maintain a small group of at least five koi in their pond. Koi can also be kept alongside community fish that can tolerate lower temperatures (64–75 F).

Before introducing new fish to a koi pond, the new fish should be quarantined in a separate enclosure for at least four to six weeks to ensure they are healthy.  

New tank mates must be introduced gradually, and keep in mind that your tank’s ammonia, pH, and nitrate levels will change when a new fish is introduced. These parameters should be monitored carefully after a new fish is added to the pond.

Koi should not be kept in overcrowded ponds, as these conditions often lead to stress and disease. 

Koi Pond Filtration 

A filter system is an essential part of any aquatic habitat. Aside from keeping ponds looking clean, filters remove harmful toxins like ammonia from the aquarium’s water and add oxygen to the water so fish can breathe.  

As koi are known to produce a great deal of nitrogenous waste, they need a powerful filter to keep their environment clean and healthy. The pond’s filter should be powerful enough to process all the water in the tank at least once every two hours. For example, a 3,000-gallon pond needs a filter with a flow rate that’s 1,500 gallons per hour (GPH) or more. If only a 1,000 or 2,000 GPH filter is available, always choose the larger option.  

When purchasing a filter, look for a filtration system that carries out both mechanical and biological filtration. Filters should be paired with a pond pump to circulate water in the habitat. Koi are slow-moving fish, so the pond should have slow water circulation to mimic their natural habitat. 

UV Filtration 

Along with biological and mechanical filtration, an ultraviolet (UV) filter will help maintain a clean environment for koi. These filters use UV light to destroy algae, which can cause pond water to turn green. UV filters need to be cleaned regularly and replaced at least once a year or more, often depending on the manufacturer’s recommendation. 

Koi Pond Water

A koi pond’s water should be tested regularly to ensure that its pH, ammonia, nitrate, and nitrite levels are stable and within a safe range. A pond water testing kit can be used to measure key water parameters quickly and accurately. Test kits have expiration dates and should be replaced once a year. 

Note: After adding a new fish or new equipment to an aquarium, pet parents should test the tank water’s quality once a week for at least two months. If the habitat’s pH, nitrite, ammonia, nitrate, carbonate, and general hardness levels are consistently in the ideal range for two months, then testing can be decreased to once a month. 

Koi Pond Temperature 

Koi are hardy, cold-water fish that do best in water temperatures between 64 and 75 F. In the winter, a pond heating system or de-icer should be used to prevent freezing. A pond thermometer should be used year-round to monitor water temperatures and make sure they do not fluctuate more than +/- 2 F in a single day. 

Koi Pond Substrate 

The bottom of a koi pond should first be covered with pond underlayment, which is a durable liner that provides an extra layer of protection against roots, rocks, sharp objects, and burrowing bugs or rodents.

A pond liner should then be placed over the underlay. Pond liners are designed to prevent water from leaking out of the pond and insulate the habitat, which is especially important for ponds in colder climates. Refer to the manufacturer’s instructions for determining how much underlay and liner you’ll need for their pond, based on its size. 

Once an underlay and liner are placed in the pond, you can add substrate (the material used on the bottom surface). A shallow layer of smooth-sided pebbles, rocks, or gravel are all suitable choices. 

Koi Pond Decorating

Automatic Feeder

Install an automatic fish feeder to feed your koi at a scheduled time each day. 

Live Plants, Rocks, and Hiding Places

Add rocks and non-toxic, non-invasive aquatic plants to your koi’s pond to provide your fish with extra shade and hiding places. 

• Healthy koi are large, active fish. Be sure not to overcrowd a koi pond with decor and leave plenty of open space for swimming. 

• Koi have a hearty appetite and may eat any plants added to their pond. 

• Water hyacinth, water lilies, floating pondweed, and water celery are all koi-safe plants. 

Koi Pond Cleaning and Maintenance

Maintain the condition of your koi pond by performing routine water changes (no more than 10–25% of the pond’s total water volume) every two to four weeks. Make sure any water added to the habitat is dechlorinated and has the same temperature and salinity as the existing water in the pond. 

Debris, leftover food, and dead leaves should be removed from the pond’s surface at least once daily with the help of a net or pond skimmer. Pond skimmers need to be checked and cleaned out about once a week to prevent clogging. Once a week, beneficial bacteria can be added to the pond to help keep its water clean, clear, and healthy. 

What Do Koi Fish Eat?

Koi do best on a varied diet of pelleted, flake, frozen/thawed, and freeze-dried fish food. Since koi need more carbohydrates than other freshwater fish to stay healthy, choose food specially formulated for them.  

Feed Koi small amounts daily, giving only as much as they can eat in one to two minutes per feeding. Because a koi’s appetite can vary, sprinkle food lightly, allowing them to eat each bite before adding more. Continue feeding for three to five minutes or until they lose interest in eating. Remove any leftover food from the pond daily, using a net or pond skimmer. 

It’s important to replace your koi’s food bag every six months to ensure freshness. If the water temperature falls below 55 F, feed your koi every few days due to their slower metabolism. When the temperature is between 55 and 70 F, feed them once daily, and if it exceeds 70 F, they can be fed twice a day.

A nutritious, well-balanced diet for koi includes high-quality commercial fish food specifically formulated for their needs, available in pellets, flakes, frozen/thawed, or freeze-dried forms. 

Frozen food must be fully thawed before feeding. 

Koi Fish Care Tips

• Treat water before adding to the aquarium
  Use a water conditioner to remove toxic chemicals like chlorine, chloramines, and heavy metals.

• Match water temperature and salinity
  Ensure that any added water is the same temperature and salinity as the existing tank water.

• Test water quality after new additions
  Test the tank water weekly for two months after introducing new fish or equipment to monitor pH, nitrite, ammonia, nitrate, carbonate, and general hardness levels. If consistently in the safe range, reduce testing to once a month.

• Replace water test kits yearly
  Water test kits expire and should be replaced annually.

• Daily equipment checks
  Inspect filters and other equipment daily to make sure they’re functioning properly.

• Filter media care

  • Replace filter media monthly or rinse it in old tank water during water changes, depending on water conditions and the number of fish.

  • Do not use hot water, bleach, or chemicals when rinsing filter media to avoid killing beneficial bacteria.

• Avoid replacing all filter media at once
  Replacing all media simultaneously can remove beneficial bacteria crucial for maintaining a stable aquatic environment.

• Remove uneaten food daily
  Use a fine-mesh fish net or pond skimmer to clear out uneaten food from the pond.

Koi Fish Veterinary Care

Annual Care

To promote the health and longevity of your koi, it’s recommended to schedule an annual or biannual checkup with an aquatic veterinarian. Regular checkups allow veterinarians to detect and address potential health issues or diseases early on, making sure your fish receive timely treatment if an issue arises.

Preventative care is key to supporting a long and healthy life for your koi. If an aquatic veterinarian isn’t available in your area, your local veterinarian can collaborate with an aquatic specialist to provide the best possible care for your fish.

While you can transport your koi to a vet, it is recommended that you find an aquatic veterinarian who will make a house call. This not only avoids transportation stress on your fish, but also gives the vet a chance to assess your koi’s habitat directly, which is especially important if there is a health problem.

Signs of a Healthy Koi Fish

• Bright coloration

• Intact fins and scales that are clean and free of debris, crusts, or fuzz

• Healthy appetite

• Swimming appropriately and active around the pond

• Full range of motion of fin movements

When To Call a Vet

• Decreased appetite

• Lethargy or decreased activity

• Discoloration or change in color of scales, lips, or fins

• Abnormal swelling, growths, or spots

• Buoyancy issues

• Fin tears or rips

• Enlarged body shape

• Staying in the bottom of the pond

Common Illnesses in Koi Fish

• Egg binding

• Koi herpes virus or carp pox

• Cancer

• Parasites

• Fungal infection

• Fin and gill rot

• Bacterial infection