A Comprehensive Guide to Koi Ponds

Koi in koi pond

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Koi ponds are a common feature in many home landscapes worldwide. Learn more about what these ponds are, their history, and how to build and maintain a proper home for these beautiful fish.

What Is a Koi pond?

A koi pond is so much more than a simple hole in the ground with fish in it. Koi ponds have a very long history, dating back several centuries in Asia. Today, they are a common feature of many yards and are designed to hold large, ornamental nishikigoi (brocaded carp), commonly called koi.

History of Koi Ponds

Descendants of the Common Carp (Cyprinus carpio) bred with the Amur carp (Cyprinus rubrofuscus), koi were kept in Asia as a food source for many centuries. In the early 1800s, carp farmers in Japan noted natural color mutations of the fish, and those fish were set aside in special ponds. Countless generations of selective breeding later, we have the colorful koi we know today. Koi were introduced to the world in 1914 at the Tokyo Agricultural Exhibition, where the farmers first brought their colored carp for public display.

Today’s koi have 15 different major varieties, including the doitsu (Japanese for “German”), a scaleless variety of koi, and the long-fin or butterfly koi, with long flowing fins and ruffled nasal folds. Koi can be very valuable pets, with the most expensive koi ever being sold in 2020 for $1.8 million.

There are koi shows all over the world, and just like shows for cats and dogs, there are strict guidelines that only the best koi can achieve. Many koi in the US are shipped directly from Japan and are held at a higher regard than domestic or US-bred fish. They are also sold for a higher price.

Common Features of Koi Ponds

Rather than a hole in the ground containing a few fish, koi ponds are practical and elegant homes for fish of all sizes and colorations. For best health, it is recommended to have at least 250 gallons of water per koi fish, and some large, breeding females may require up to 500 gallons per fish, due to their massive bulk when full of eggs in breeding season.

In addition to the main pond, all koi ponds require an efficient filtration setup. This is a combination of mechanical, biological and other filtration components. These types of filters include:

Mechanical Filtration

These stages trap solid particulates from the pond water, which can clog biological filtration if not removed. This may include a skimmer basket, settling tank, open-cell foam sheets, sieve filter, or drum filter. The mechanical filter needs to be frequently cleaned to remove trapped wastes to keep it working properly.

Biological Filtration

These components house beneficial bacteria that remove ammonia and nitrate from the nitrogen cycle. The most common component in many koi ponds is a pressurized bead filter. These filters contain many small plastic beads with high surface area for holding the good nitrifying bacteria. These filters needs to be backwashed weekly to prevent compaction. Other biological filtration options include running filter water through gravel beds, lava rocks, bakki showers, plastic matting, recycled strapping, or other porous, non-degrading materials. Be careful not to overclean these components with chlorinated water, hot water, or a pressure washer, as that can remove the beneficial bacteria that grow on the surface of the media.

Other Filtration

Most koi ponds should include UV (ultraviolet) filtration. This filter consists of a UV bulb contained within an opaque housing that the pond water is pumped through. This is used to zap algae and other biologic particulates. These UV light bulbs will need to be cleaned periodically and replaced every year.

How to Build a Koi Pond

Although many homeowners are fully capable of DIY koi pond construction, consulting with a professional builder is always recommended. Planning all your components and layout is critical to achieving a successful koi pond. Start by determining how many fish you want and work backwards from there, provided your yard has enough room. Or you can start with your desired koi pond size and choose corresponding components. Koi should never be kept in a fish tank; it is
too small for koi, most of which can top out at 24 to 36 inches in length. Some large females have reached over 40 inches in length!

When selecting filtration, it is best to overestimate your total pond water volume and choose components that are slightly larger than required for your pond. For example, if your pond is 4000 gallons, choose a filter rated for a pond of 5000- to 6000-gallon capacity. This will provide you a little wiggle room as your filtration is being established and if you happen to forget any regular maintenance. The flow rate from the water pump through the filter should be high enough to pass the total water volume through it every two to four hours. So a 4000 gallon pond should have a water pump that pushes water through the filter at 1000 to 2000 gallons per hour.

What Types of Fish Live in Koi Ponds

The most traditional occupant of a koi pond is a koi fish (Cyprinus rubrofuscus). Other good choices for a koi pond is their close cousin, the goldfish (Carassius auratus). It is a myth that koi and goldfish cannot be kept together. Given their phylogenetic relationship, they share all the same bacteria and parasites, and even some viruses, such as Koi Herpes Virus (KHV). Fancy goldfish with the rounded body shapes do not do well in koi ponds, given how much swimming is required, so stick to comet goldfish for ponds. Other koi pond inhabitants may include common catfish, provided they can't wrap their mouths around any koi, and high fin sharks.

Koi Pond Maintenance

There are daily, weekly, monthly, seasonal (usually twice a year), and yearly tasks that must be performed to maintain your koi pond. Please note: these are general guidelines only. Your pond's tasks may vary depending on your filtration, size of the pond and number of fish.

Daily Tasks

  • Feed the fish, observe them while they eat
  • Evaluate fish swimming behavior and appetite, and monitor them for illness and injury
  • Remove excess food and debris from skimmer basket
  • Monitor water level and top off if necessary with dechlorinated water
  • Check water temperature

Weekly Tasks

  • 10% water change using backwash operation in filter cleaning, or with a pond vacuum
  • Perform water quality testing if new fish or equipment have been added within last 2 months
  • Remove dead leaves from any live plants and debris from pond bottom

Monthly Tasks

  • Perform water quality testing if no new fish or equipment have been added within last 2 months
  • Move substrate around with long rake or pole to release trapped debris
  • Shake biological filtration media or rinse gently in a container of pond water
  • Check filter system, UV light and water pump for proper operation

Seasonal Tasks

  • Pond deep cleaning to remove excess algae and debris
  • Trim dead leaves from water plants in the fall
  • Check water bill and see if any increases could indicate pond leaks
  • Prepare filter system for winter in areas where the pond may freeze

Yearly Tasks

  • Change UV bulb
  • Replace liquid-based test kit

When built and maintained properly, koi ponds can be a great home feature. Although not "maintenance-free" pets, advance planning goes a long way in ensuring your koi pond is a success.