What Do Koi Need to Thrive?

Koi, also known as jinli or nishikigoi in Japanese, refers to the specially bred and colorful varieties of the Amur carp. The hobby of keeping and breeding koi began in the Niigata Prefecture in Japan in the 19th century. After an exposition in Tokyo in 1914, the entire world took notice of the particular beauty these fish can bring to an aquatic garden. Today you can choose from dozens of different colorations, patterns, and forms. Modern koi often feature fantastic combinations of red, orange, yellow, white, black, blue, and cream, depending on the fish and its lineage.

In Japanese, “koi” is a homophone for another word that means “affection” or “love”. This name was chosen because it describes how these fish are sometimes given as gifts and speaks to the nature of koi in general. They are docile, social fish that enjoy living in pairs or groups. While goldfish will learn to respond to feedings, koi actually become friendly with their caregivers and are more like pets than most other fish. Koi will not eat other fish or fight with one another. Some koi have even grown to enjoy human contact and will swim up to the surface of the pond to be petted.

As you dive into the fun world of koi, you'll discover that with the proper tools to keep your pond clean and your koi healthy, these fish can be your companions for 20 years or longer.

Plenty of Clean Water

Koi grow to be quite large with proper care; typically reaching a maximum size between two and three feet in length. Some varieties even reach four feet after a few decades. Not only are koi long, but they are also heavy. When full grown, koi weigh an average of 35 pounds. It is important to plan your pond according to the needs of adult size of koi instead of their size at purchase. When estimating, you will need, consider that each adult koi will need between 500 gallons (for males) to 1000 gallons (for females) of water. These water estimates equate to approximately 30 gallons of water per one inch of fish.

Koi need a high-quality water, with low rates of ammonia, nitrites, and nitrates. Unfortunately, their own waste contributes plenty of these compounds to the water. Another important factor in your koi pond's water quality is pH, which should be maintained around 7.5-8. A good way to create a buffer for pH changes in your pond is to add calcium carbonate. This additive is sold as gravel, rocks or powder as “agricultural limestone”. The addition of salt up to 5 parts per thousand (.5%) also assists in keeping health problems at bay, yet, be aware that a lower salinity level is necessary in winter.

Lots of Dissolved Oxygen

Fish breathe by taking water in through their mouths and passing it through their gills; dissolved oxygen in the water is transferred from the thin walls of the gills to the fish's blood. Water can be aerated with turbulence, such as a waterfall feature, but many koi ponds utilize both air stones and additional air pumps in the filtration area to provide fish with enough oxygen. An electric air pump pushes air to a diffuser or air stone that creates tiny bubbles, transferring the maximum amount of oxygen into the water. Signs of oxygen depletion in koi include:   

  • Koi staying around air stone or other oxygen source
  • Slow or sluggish swimming
  • Fish gasping for air at the surface
  • Pale gill color
  • Lack of eating

Koi, like all fish, require more oxygen in hotter temperatures when they are more active. Keep in mind that air pumps, tubing, and air stones will degrade over time, thus no longer providing as much oxygen. So, it is very important for these components to be replaced as they wear out.

Relatively Warm Temperatures

While koi can survive in water temperatures ranging from 34 to 90 degrees Fahrenheit, your koi will do best when their water is around 65-68F. Frequent changes in water temperature are not healthy for koi. In summer months, shade your pond from strong sunlight or algae blooms may take over. Trees can help provide shade, and pond netting or tents are helpful for a bit of shade and temperature control.

Heating your pond will create an environment that is much less stressful for your koi in the winter and prevents ice from forming on their gills, which can be deadly. You may choose to keep your pond at 65-68F and have active happy koi, though this will very expensive in colder climates. At 62F, you can save money and still have fairly active fish. A pond temperature around 50F will prevent health issues and ice formation while keeping the fish in torpor. In spring you may gradually increase the temperature by no more than 5 degrees per day until you reach a temperature where they are active and can eat.

Having the ability to raise the pond temperature can also save the lives of your koi should they get sick.

When selecting a heater, there are important factors to consider. A good heater is one that you can afford to run; make sure the efficiency rating is as high as possible. Dependability is just as essential. A gas heater can keep your koi alive if your power goes out in the winter and can be run with a small generator. Make sure that the heater you are purchasing does not use copper in its heat exchanger, as copper is toxic to koi.

Vegetation to Hide in and Eat

Plants improve the koi pond in a number of ways. They release additional oxygen, soak up extra fish waste, provide shade and shelter, and produce food for your koi. If shading a small pond with plants, aim for 70% surface cover. A larger pond will remain cooler in the summer and does not require as much shade from plants. There are numerous types of plants that coexist with koi, but they vary in cost as well as ease of care and speed of growth.

You can opt to keep your plants behind wire or mesh if you wish to deter your koi from eating the root systems, though this will prevent the fish from receiving the benefits of having places to hide or escape the sun. Some people let young plants grow this way and then transfer them to the main part of the pond later once the plants are large or plentiful enough to endure koi nibbling.

Appropriate Water Depth

Adult koi need 500-1000 gallons of water per fish, but you can’t simply make a very large pond that’s only a foot deep. Koi need a minimum depth of 3 feet to thrive, with up to 8 feet preferred by most hobbyists. Greater depth gives the koi the ability to move vertically for exercise but makes the pond more difficult to clean and maintain. In addition, increased depth can also complicate the catching of koi should the need arise. A shallow pond is more prone to temperature changes caused by sunlight and to oxygen loss on hot days, while a pond that is very deep without much surface area may be very cold on the bottom and difficult to heat. Unheated ponds must be three feet or deeper to leave space for the koi to stay under the ice. Once you’ve planned and dug your pond, the best way to maintain water levels and prevent seepage into the surrounding soil is to use a koi-safe liner like AquaArmor by BTL Liners which is plant and fish safe as well as resistant to punctures, tears, and UV degradation.       

Open Water Surface

An open water surface is important for maintaining the health and longevity of your koi. While you may have plants to provide shade covering about 40-70% of your pond surface, be sure to keep other floating debris from obscuring the rest. Installing a skimmer makes this work nearly automatic, but a pond net is valuable for objects too large or bulky for the skimmer to collect. In the winter, do not let your pond freeze over entirely. A heater may prevent this issue, but if your pond does not include one, you must create a hole in the ice for fresh air to enter and for gases to escape the water. Some hobbyists choose to buy a deicer that keeps the surface temperature above 40F. If your pond does freeze over, don’t just stab at the ice to break a hole--you are likely to damage your koi’s lateral line, an organ that detects pressure changes in water. The safest method is to pour hot, dechlorinated water in one place to melt a hole.       

Nutritional Food in the Right Amounts 

Just like us, koi thrive when getting proper nutrition in appropriate amounts and when they enjoy the taste of their food. Koi are omnivorous and will eat plants, algae, bugs, and animal matter. Balanced nutrition can come from pellets typically made from wheat germ and essential nutrients, but you can also supplement this with fresh foods like peas, lettuce, fruit, and squash. Automatic feeders may seem helpful, but the needs and appetites of koi change with the seasons and varying temperatures and will require adjustments. Overfeeding leads to rapid water quality changes, while feeders that jam can leave your fish starving. In all weather, be sure to only feed your koi around 3-4% of their weight per day. Food in excess of these recommendations will create an unbalanced cycle of waste and can make the water toxic. Floating-type pellets allow you to better see your koi as they come to the surface. This allows you to check for damage in addition to enjoying their appearance. Sinking pellets, however, are more likely to be eaten and can lead to better growth and brighter coloration. Many hobbyists combine types for the benefits they each bring.

Occasional Pond Cleaning

Even though your koi live in a pond and not an aquarium, weekly water changes are necessary for keeping the cycle of waste balanced. If your pond is greater than 20,000 gallons, every other week will be sufficient. Drain 10% of the pond’s water and replace it with dechlorinated water. Smaller ponds, or larger ponds in warmer weather when the fish are eating more, may require a 20% change. This is a good time to check for any debris you or your skimmer may have missed, such as a dead fish or other wildlife. Below the surface, a thin buildup of bacteria sediment or sludge is normal, but if the buildup is an inch or more thick it can make the water toxic to your koi. Depending on your pond, you may need to remove this sludge annually or even seasonally. There are various methods that can be utilized, such as using a net to scoop large sunken debris that has not yet decayed, purchasing sludge digesting bacteria, using a pond vacuum, siphoning water from the bottom, and if all else fails a complete pond drain and restart during which you will need a place to temporarily house your fish.

Other seasonal maintenance includes winterizing your pond. Once the weather drops below 50F, turn off any waterfalls or fountains to keep them from losing heat from the pond or freezing over. You may also need to reduce aeration or move the location of air stones. Make sure sludge isn’t so high in the bottom that the fish will have trouble staying below ice sheets that may form on the surface and schedule a fall cleaning if necessary.

From a small collection of carp in Japan to an international phenomenon, hobbyists around the globe have created a rainbow of possibilities in each unique koi. With this guide to designing a pond that meets the needs of the koi while complementing your landscape, you’ll enter the hobby the right way and avoid many frustrations and unnecessary costs.


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