How Small Can a Koi Pond Be?

A Koi pond can be a charming and attractive addition to your home, either as an outdoor yard feature or decorative indoor water garden. These “living jewels” and “swimming flowers” are known for a gracefulness in the water that most find people find relaxing and meditative.

How small can a koi pond be? Not very small. At least 1,000 gallons, and no less than three feet deep. A watery home for Koi needs to be large enough to accommodate some rather large fish. Adult koi can grow quite large and need ample space to swim.

While 1,000 gallons might sound like an overly big pond, keep in mind that while a few, six-inch koi may find that size pretty spacious, within four years, each of those fish can quadruple in size. Before long, among the adult koi, swimming space will be very tight.

Why Do Koi Need So Much Space?

While the average healthy goldfish can grow to four inches long, that’s just about where a koi begins. And once a pond is built, it’s an incredible undertaking to make it bigger down the road. So, it’s best just to start as large as you can go.

Here are some reasons why these colorful and portly beauties have such a need for depth and volume of water.

Koi get bigA fish that is even a foot-long needs quite a bit of space to spin around and change direction, let alone swim enough to get proper exercise.
They can live for a very long time. The average lifespan of a koi is 20-30 years. They’ll be around for a while. And they keep on growing.
Colder zones need a deeper pond.
In winter, a Koi pond needs more depth to keep from freezing solid, if the fish are to survive.
They don’t react well to rapid temperature change.The smaller the volume of water, the greater the speed at which it heats up or cools down. If a pond is too small, extreme heat and cold can quickly affect the koi.
There are predators in your yard.
A shallow pond makes it too easy for ducks, hawks, raccoons and other wildlife to reach the fish.
Big fish generate big waste.
Koi ponds require very regular cleaning as it is. A pond that is too small will be a real struggle to keep clean. Koi waste can add up fast.

Determining What Size Pond to Build

If you’re considering a koi pond, start by thinking about how many fish you’d like to keep in it. And then double that. Keep in mind that you might end up with more fish than you start with. They’re really pretty and you might want more down the road!

Just how big is a pond that holds 1,000 gallons? At the minimum suggested depth of three-feet, a 1,000- gallon pond would be approximately 6 feet wide by 8 feet long. And that size pond will comfortably house just five, average-size koi. More fish, bigger pond. So, if you approximate five fish to every 1,000 gallons, you can estimate what size pond you will need, depending on the number of koi you want to keep.

Don’t want a rectangular Koi pond? If your pond holds enough water, you can make it any shape you like. To determine how many gallons you’ll need – or the volume of your pond – multiply the Length x Width x Depth of your shape. Clearly, this won’t work as easily for ovals or odd shapes.

This is when an online pond size calculator, like this one, comes in handy to do the math for you.

Maintaining an Outdoor Koi Pond

Once you have that much water – and several new koi – to take care of, it’s important to know how to keep the underwater environment and the fish healthy. As you might imagine, this involves some regular, daily maintenance year-round – and the proper equipment to make it go smoothly. While the following isn’t an exhaustive list, it will give you an idea of what is involved in the daily maintenance of your Koi pond.

What You Need to Do
Supplies That Help
Keep your water clean
Pump and filtration system
Bottom drain
UV sterilizer
A long-handled net, skimmer
Plants around your pond
Proper temperature, between 59-77° Fahrenheit
Temperature gauge
Aquatic heater
Aerator or fountain
Plants around your pond
Healthy oxygen levels
Aerator or fountain
Plants around your pond
Keep water from freezing in winter
Keep Predators away
A fence or pond-netting
A “predator statue”
Motion sensor

What About an Indoor Koi Pond?

An indoor koi pond can be a gorgeous focal point in your home. And, in colder climates, it may be a great way to ensure healthy koi all year long with less worry and expense.

But an indoor pond still needs to be as big as you can possibly fit in your home.

Most likely, your indoor pond (unless it is in a commercial space with ample square footage), will be smaller than the recommended size mentioned above. Not many homes could accommodate a 1,000-gallon pond. Because of the smaller size of indoor koi ponds, maintaining excellent water quality and avoiding overcrowding are critically important measures.

Avoiding Overcrowding

Even indoors, your koi will still need room to swim and grow, so staying away from overcrowding is as important as ever. According to Pond Trade Magazine, the very minimum allowance is 10 gallons of water per one inch of fish. Or one, five-inch koi per 50 gallons of water. By this calculation, if you have a 300-gallon pond in your home, you can accommodate six, five-inch fish.

Keep in mind, those baby Koi will grow much larger! So, you are better off thinking in terms of 10-inch fish. In this case of the 300-gallon indoor pond, you would bring in three koi. Otherwise, to avoid overcrowding, you may have to give up adult fish as they outgrow the space.

Keeping a Healthy Indoor Environment

With less water volume, keeping healthy water levels in an indoor pond is crucial. Just like an outdoor koi pond, you’ll have to take steps to promote and maintain water quality, oxygen level and water temperature. In addition, because the pond is in your home, you’ll want to take additional precautions to help prevent damage caused by leaks or humidity.

Some of the things you’ll need for an indoor koi pond:

A high-quality water pump that can filter the entire pond in a few hours
An air pump to maintain a proper oxygen level in the water
Aquarium heater to help regulate temperature, when needed
A drain, if possible
A fan to disperse heat and moisture
Good natural light or incandescent lighting, especially if you have pond plants, too.
A water detector or leak alarm, just in case

Is it Expensive to Purchase and Keep Koi?

Koi fish can be very expensive. From pet shops to breeders, the price of Koi can range from $10 to upwards of $1,000, depending on what type of koi, how large they are and even how old they are.

Koi have many classifications, usually recognizable by colors, patterns and body shape. Some Koi are very carefully bred for certain characteristics and qualities, from specific color patterns to a particular gracefulness in the water.

According to the Business Insider, the most expensive Koi sold for $1.8 million in 2017. But some more common types of Koi are even available inexpensively online. Depending on the size of your pond and the type of koi you pick, stocking it can be a pricy endeavor. But, when maintained correctly, your new pond is sure to bring you years of fulfillment and relaxation for years to come!

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12 thoughts on “How Small Can a Koi Pond Be?”

  1. Oh, this is charming. Thank you for providing so much detail and a realistic guide for what size pond to have, even indoors. After reading this, I might go with a smaller tank indoors. I’m sure you’re helping the Koi population by realistically informing us on how to set up and maintain a Koi pond. 

    • Thanks. Keeping a tank indoors is just fine. It’s neat to think about how you are not only helping people, but giving wildlife a chance at getting a loving home, too. I hope you take the plunge someday.

  2. Kosi ponds are really awesome and those people who keep them indoor are the real deal. I don’t think I can do that. A friend of mine is looking to start his own koi pond and he just wants something really small to start with. I’m sure if he reads this post, hell have an insight of what to do. Getting koi can be really expensive and I agree with you but at the end of the day, it can be some good investment.

    • By all means, then he can get started and who knows it may take over his life. However, you mentioned starting small. I don’t know if koi are right for him then because once they grow they can get to around 3 feet. You also need a 1500 sq. foot pond to have enough room for them comfortably. Not trying to scare you off, but trying to manage your expectations. Thank you so much for reading.

  3. Excellent article on How Small can a Koi Pond Be? I’ve learned greatly from your post,You have opened my eyes to things needed to be done,how to go about it, the condition,atmosphere,size to be considered in setting up a pond for real profitability,what really surprised me is the life span,the average lifespan of a koi is 20-30 years. They’ll be around for a while. And they keep on growing.It is amazing,thank you for this educative and informative write-up.

    • Keep em healthy and they will live a long time. Unbelievably, one record koi lived to 225 years. While not the norm that just blows my mind that something lived during the Revolutionary Period. It died back in 1975 I believe. Mant people love to raise and sell koi. It is a popular business. 

  4. I must commend this post for not just giving me a full education but also giving all necessary basics on how to plan for a fish pond and and also how to  maintain them is also pretty cool to note that there are a lot of  koi fishes that were really good enough to keep at the pool for me.. This is a really great post  that shouldn’t be ignored at all because of its factual information. 

    • I agree. I like to lay the facts before the people to let them know what they are getting themselves into starting a koi pond. Maintaining the ponds can be rough at times, but it needs to be done nonetheless. Thanks for your response and have a nice day.

  5. What an excellent article. I appreciate all the information you have provided to get us started. My husband and I have been considering adding a koi or goldfish pond to our back yard and as of yet had not taken the time to research and find what we might need. I am so glad I found your article. It has given us a starting point to make some plans for next summer. How deep would we need to go for Canadian winters? Would we need a floating type deicer? 

    • I suggest in the article at least 3 feet, but that’s the minimum. If it’s going to be a really cold winter maybe go at least 5 feet. You can’t make them too deep. And when ponds freeze over the gases and toxins get trapped in with the fish and they slowly suffocate. Therefore, you do needs a de-icer. I hope you have an awesome day.


  6. I have seen a beautifully koi pond before. Looking at it brings serenity and inner peace. Its vibrant colors are surprisingly relaxing and pleasant. They clean their koi pond and feed the fishes once a day to avoid decay from uneaten food. My chinese friend says that keeping koi fishes as pets bring good fortune, success and longevity of life. This also one of the reasons why they have it properly maintained because they don’t like their symbol of good luck to be marred with dirt and muddy waters. 

    I enjoyed your in depth article about maintaining koi fishes in their indoor or outdoor habitat. It gives substantial information on spacing and measurement, expenses, supplies, and keeping the fishes healthy. Your website is a “go-to” when it comes to koi fish expertise. Just a question, is it necessary to have plants and tank decoration in the koi pond?

  7. The Japanese generally believe the koi represents good fortune to their keepers. They have a lot of lore about them. I think it is just a beautiful fish. The vibrant colors are appealing and I just also happen to be fascinated by the Japanese culture.

    Plants have many important purposes besides just decoration. They provide shade for koi during the hot times of the day. Lilies and such plants also provide oxygen in the water at night. They will get eaten by the koi too since they are omnivorous fish.

    Have a nice day


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