What To Know About Backyard Koi Ponds

Updated: Mar. 22, 2024

Do you have backyard koi ponds on your water features Pinterest board? These insider insights will help you decide if one is right for you.

For Cheryl and Francis Vetter of Wadsworth, Ohio, their backyard koi pond, surrounded by annuals and perennials, catches a visitor’s eye as soon as someone pulls into their driveway.

“We created our pond 27 years ago to have a place of peace and entertainment,” Cheryl Vetter says. “There are lots of wonderful moments at the pond and fire pit with family, friends and fish.”

What Is A Koi Pond?

As the name implies, it’s an outdoor pond populated by koi fish, a colorful, domesticated carp that adapts well to a variety of spaces. Originally domesticated in China, these beautiful fish endeared themselves within Japanese culture. Rice farmers in Japan began keeping and breeding them in the 19th century.

Choosing Fish for a Backyard Koi Pond

With more than 100 types of koi, there are many options of vibrant colors, ranging from oranges, blues, yellows and whites. Sizes run from a couple of inches long to three feet. Most backyard koi species range between eight and 14 inches.

Cheryl Vetter notes it’s possible for koi to live up to 100 years, although 20 to 30 years is more typical.

Besides koi, goldfish, another type of carp, thrive in backyard ponds and live peacefully with their colorful cousins. They’re easy to keep, though lacking the koi’s charisma and the splashes of color.

Note: Never add captive frogs to your backyard koi pond; these may expose native frogs to disease. You may find that native frogs move in on their own.

Planning Your Backyard Koi Pond

While there are traditional Japanese-style koi ponds, artistic expression and functionality intersect in creating an ideal habitat. Small, simple ponds ranging from $50 to $130 per square foot. The cost increases with greater size and elaborateness.

Consider these items in your planning:

Check local regulations for backyard koi ponds

Contact your local fish and game or natural resource office before you dig. Some states require a koi pond to be at least 200 yards from a river or stream, and not within the 100-year flood plain. If the area floods, the koi would find their way into natural waterways — not optimal.

Also, find out whether you must have a fence around the pond for safety.

Figure the depth of the koi pond

Backyard koi ponds tend to run deeper than typical backyard water features. A standard depth is four feet. Cheryl Vetter says their 30- by 6-foot pond ranges from 22- to 26-in. deep.

Calculate the number of fish

Plan the size around for the number of fish you’d like to keep. The rule of thumb: One koi per 250 gallons of water. For a pond 6- by 8-ft. and four feet deep, you could easily keep five or six koi.

How To Build a Backyard Koi Pond

Japanese garden with koi-pondfotolinchen/Getty Images

Once you’ve done the basic planning for your backyard koi pond, it’s time to grab a shovel or hire a pro who will likely bring in a backhoe.

Lining the pond

After digging the pond space, it’s time to line the pond. Easy calculators on pond supply websites can help you figure out the amount of material required.

You’ll need an underlayment to protect the main liner from rocks or other sharp objects from below, as well as at least a 40-mil fish safe flexible liner like AquaArmor. Rigid pond liners, will eventually crack and limit the pump and filter options.

Installing the pump system

The pump system is the heart of the backyard koi pond. Some of the best involve a bottom drain or skimmer pump that pulls the water through a series of filters before recirculating it back into the pond. Adding an aerator to increase the oxygen in the water is also beneficial, particularly in smaller ponds.

Fill your pond

You can fill your backyard koi pond with standard tap water, but it’s important to know what’s in the water and how to treat it.

Chlorine is a common element in tap water. It can be mitigated by allowing the water to sit for several days so the chlorine evaporates, or by using a chlorine treatment.

A more problematic, yet common, chemical is chloramine. Some chlorine treatments remove or minimize this substance. You could also install activated carbon filters to deal with standard water chemicals and other contaminants.

Koi thrive in water with a pH between 7 and 8.6, tested with a simple kit found at garden stores. Because ponds tend to be acidic, adding limestone sometimes boosts the alkalinity of the water enough. Koi thrive in temperatures from 59 to 77 degrees, so if it’s too cold, use a heater. Deep pools and shade coverings help if temperatures rise too high.

Landscaping your backyard koi pond

Decorate with plants and rocks. Plants naturalize the look and provide protection for the fish, reducing algae and acting as natural filtration. The rocks provide a strong pondscape structure.

Backyard Koi Pond Maintenance and Protection

Remember to add water

Depending on your location, up to 15% of the water in your koi pond will evaporate each week. Add water every seven to 10 days.

Tending to the filters

Skimmer filters capture leaves and other debris and deposit them in an easily-cleaned basket. There’s a filter pad between the collection area and the pump that puts the water back into the pod.

“Pads for the skimmer [filter] should be changed every other day, especially during the summer,” Cheryl Vetter says. “Don’t wait until the pump is sucking air.”

Skimmer filters capture the larger debris before it clogs anything else. Unless you have a lot of leaves or living plants, cleaning the skimmer once a week is most likely sufficient.

Tend to the separate biological filter, which removes the smaller particles that decrease water quality, two to three times per year. Remove the fine filter pads and rinse off any sludge to keep the water fresh. Many pond owners conduct a spring and fall rinsing and, if weather permits, a mid-winter cleaning.

Keeping koi warm in the winter

Koi are durable fish, but they have their limits. When the water temperature drops below 50 degrees, they’re more susceptible to pathogens. Depending on your location, a constant heat source might be required. Because copper can harm koi, choose a heating unit without it as part of the heat exchange.

In areas of the country where the temperature stays below freezing for extended periods, koi owners bring their fish indoors and keep them in large aquariums. The greatest challenge is providing enough space for the fish.

Protecting koi from predators

Beware of blue herons. “Our neighbors were surprised when they spotted [them] by the pond,” says Cheryl. “They lunched on a few fish and frogs.”

Cormorants are another bird with an eye for fish. If they arrive in a group, they can wipe out a backyard koi pond’s population in short order. Raccoons, along with mink and weasels, are also koi predators.

Adding Nycon Koi Kastle Fish Cave Shelters gives fish a place to protect themselves. You can also install decoys that resemble coyotes, swans or an alligator to dissuade potential predators.