Building and Maintaining a Koi Pond

Author: Marijn van Haaren

A pondkeeper explains how to set up and maintain a pond that is ideal for koi (Cyprinus carpio) in this helpful guide.

What to Know Before Building a Koi Pond

A koi pond differs greatly from any other. Ponds are purpose-built habitats, and creating and maintaining a koi pond can be a time-consuming and expensive process. Koi carp are brilliant fish to own, and all your hard work and money will be well spent when your pond is fully functioning. Sadly, digging a hole, throwing in a liner, and adding a few fish simply isn’t enough. Here I will cover exactly what you need to do to make sure you get the process right the first time.

If you were simply to introduce koi into a pre-existing pond, the result could be disastrous—unless, by happenstance, that pond meets all of their needs. Many people are misled into thinking that because koi can survive for a lengthy period they are particularly hardy fish, but this is untrue. Koi may be resilient, but they cannot thrive in a habitat that is wholly unsuitable. No matter how well you maintain your pond, your koi will not survive if it’s not appropriate to their needs, so it’s best to start your pond from scratch. To help you with this process, and to ensure that your koi have the greatest possible chance of survival, I’ve put together this guide.

Plan for Perfection

The first thing any prospective koi owner must realize is that keeping koi isn’t an exact science. There’s no set formula for success, and even experienced keepers run into problems. Here I’ll draw upon my personal experience in keeping koi, giving you general ideas about what worked for me. When researching ponds and seeking koi-keeping advice, you’ll encounter a great deal of conflicting information about the best practice methods and the best way to start your adventure. To prevent any conflicting advice from sabotaging your efforts, it is wise to stick with one method and not jump back and forth between them.

Before you start buying products or planning your pond, I would advise that you go out to see some ponds in your local area. Most pond owners will be more than happy to show you the fruits of their labor, and they can give you invaluable advice about how much things cost, the implications of making certain decisions, and how you can source relevant materials. Only after you’ve done this exploratory work can you really begin to understand the scale of the project at hand and how it fits into the budget you have available.

Consider Your Limitations

The main problem that pondkeepers often have is deciding the size, scale, and design of their pond. It’s all too easy to be overly ambitious in this regard, and you really must consider your limitations carefully before you start buying anything. Also, although many fishkeeping enthusiasts want a water garden and a koi pond, this is unachievable for most of us because of the cost and upkeep.

Many hobbyists start by adding a few koi to a small pre-formed pond, and although this has its merits (providing vibrancy and color), it is also very limiting and never allows the fish to reach their full potential. The main drawback of this approach is that the water temperature and pH of a small pond can fluctuate rapidly, causing great harm to the fish that inhabit it. Other unavoidable side effects are that waste buildup will limit the size of the koi that inhabit the pond and that the shape of the fish will likely be hindered by a lack of swimming space.

So let’s take a closer look at exactly what is required for koi to flourish properly and how you can overcome any temperature and pH fluctuations as well as any filtration issues.

Determining Pond Size, Shape, and Depth

What size, shape, and depth to make your pond is your first big decision. Leaning on personal experience, I would estimate that the perfect size for a koi pond is 25 feet long by 13 feet wide with a maximum depth of 3 feet (7.5 meters long by 4 meters wide with a depth of 1 meter). Ponds can be smaller than this, but you really shouldn’t stray too far from those measurements. If you do, your koi will struggle to flourish. It’s especially wise to limit the width of your pond to no more than 13 feet (4 meters) if possible, because anything larger would make it more difficult to catch the fish if necessary (and experience says that it will be necessary at some point).

Calculating Water Volume

Using these measurements, it is also simple to work out the volume of water in your pond (a measurement that is vital when you’re planning how to alter the water’s pH levels). To calculate your pond’s volume in gallons, you simply multiply length x width x depth x 7.5 gallons per cubic foot. So, taking the above measurements, you would multiply 25 x 13 x 3 x 7.5, meaning that the volume of water in your pond would be 7313 gallons (27,683 liters).

Think About the Future

Although it’s incredibly tempting to economize by building a smaller pond, doing so may put you in the position of having to build additional ponds to house your growing collection in the future. Most enthusiasts who start off small end up building more ponds later and therefore spending more in the long run. You’re better off starting with a pond that seems larger than necessary at first, as this ensures your koi will have room to grow even if you add more at a later date.

Always remember that the comfort of the fish and the sustainability of the pond are the most important considerations. Despite interbreeding and natural variation, koi are far less versatile than goldfish, and they struggle in conditions where goldfish thrive. To ensure that your koi exercise properly, stay healthy, and avoid stress, you need to make the size of your pond a priority, building one as close to the ideal dimensions as possible.

Constructing a Koi Pond

Once you’ve got the size and scale of your pond sorted out, you need to begin planning the actual construction. Most of the construction techniques you use will be dictated by the desired shape of your pond.

Ponds should be designed to avoid what are known as “dead spots,” where water doesn’t circulate properly, because these spots can cause lasting health issues resulting from the buildup of toxins. To maximize circulation, people often use concrete blocks that are rendered and finished with fiberglass, which allows the corners to be rounded off and the pond to slope correctly, with water flowing toward the bottom drains. Properly circulating water remains oxygenated, so fish can flourish no matter where they are in the pond and there are no dead spots to harbor disease or allow plant life to alter the pH.

Unfortunately, despite being the best construction method, this method is also the most expensive and may prove to be unaffordable for many hobbyists. If this is the case, there is no shame in resorting to a rubber pond liner and digging a hole. Just be sure to monitor the situation carefully and be aware of the issues that we will now discuss.

What Makes Good Water Quality?

Good water quality is the main factor in successfully keeping koi, and pH, ammonia, nitrite, and nitrate levels in the water should be checked regularly. Ideally, the ammonia and nitrite levels should be zero and the nitrate level should be kept below 50 ppm, but the decomposition of excessive organic matter (e.g., as a result of overstocking and/or overfeeding) can cause these levels to rise too high, so they must be monitored closely.

With regard to pH levels, anywhere between 7.0 and 8.0 is acceptable, but you want to aim for 7.5. Anything below 6.0 can lead to acidosis disease, and anything above 9.0 can cause alkalosis. As an additional point: when you’re adding water to your pond, never use straight tap water, as it contains toxic levels of chlorine or chloramine. It must first be treated to neutralize those chemicals.

Your pond’s water parameters should be monitored regularly using testing kits. If testing reveals any issues, then you should carry out a water change as quickly as you possibly can. Changing the water will lower the level of dissolved pollutants, but you shouldn’t get carried away trying to change the entire supply. Changing more than 50 percent of the water in the pond will shock the fish, causing further problems. When changing the water in response to a water-quality problem, it is advisable to avoid feeding the fish, as this will help you isolate the cause of the problem. Overfeeding and overstocking, especially while the biological filter is still maturing, are the most common causes of water-quality issues in koi ponds, so be careful.

Minimizing dissolved pollutants and maintaining a constant pH level aren’t the only water-quality issues you will face; you must also ensure that your water is at the correct temperature. A water heating system is preferable for koi, but it may not be essential. If you live in a cold region, the koi will hibernate underneath surface ice if necessary. It is possible for them to live in this fashion for a number of months, but it is not their preferred way of living, so you should seriously consider a water-heating system that will maintain the pond at more ideal temperatures for the koi’s well-being and growth.

Filtration: Keeping Your Pond Water Clean

In order to keep the water quality at the appropriate level discussed above, you’ll need a koi pond with a bottom drain that continuously gravity feeds to the filter system. Continual gravity feeding allows for the purging of anaerobic sediment, which poses a threat to the sustainability of the fish, while leaving most of the big stuff fully intact in the settling phase of the filtration system. By using gravity-feeding systems you ensure constant water circulation, and this means that your fish are less likely to succumb to disease because of the presence of the harmful chemicals discussed above. Of course, a bottom drain alone simply isn’t enough; you’ll also need an adequate filtration system that incorporates the following elements:

1) Settling Chambers

A vortex/whirlpool is the most efficient settling chamber design. Water will enter this system on an angle two-thirds of the way down the container, which will, in turn, create a swirling motion that forces the larger pieces of debris to the sides. After this, gravity will draw them down to the bottom of the pond, where the purge line will enter the purpose-built container. This makes cleaning simple because when the debris has built up to a sufficient level, you can easily pull the knife valve to get rid of the waste. For most koi ponds, this container should be a minimum of 40 inches (102 cm) in diameter and 40 inches deep.

2) Mechanical Filtration

A mechanical filtration system will extract suspended debris from the water as it passes through the filtration medium. To be effective, it must be situated directly in the path of the water flow. Cylindrical brushes (around 4 inches [10 cm] in diameter) work best and, ideally, you want them to be made of a steel core and nylon bristles. To ensure maximum exposure, you’ll need around four rows of brushes, and you’ll want them to overlap slightly. They will also need to be cleaned regularly to ensure proper functionality.

3) Biological Filtration

Biological filtration allows beneficial bacteria provided by Mother Nature to convert harmful ammonia and nitrite into less toxic (at lower levels) nitrate. The population of these helpful nitrifying bacteria must be sufficient to keep pace with the decomposition of fish waste, uneaten food, and other organic material in the pond, so this is a delicate balance to maintain. That’s exactly why overstocking and overfeeding a pond are among the least helpful things you can do as a koi owner. To see whether biological filtration is working effectively in your pond once it has been stocked, you’ll need to make regular use of simple, inexpensive, and effective test kits.

Choosing the Right Filter Size

The size of these filter systems is crucial, and you’ll need to consider a number of factors when determining what will work best for you. Most filtration systems will state a size of pond they’re rated to filter, but be aware that this doesn’t tell the entire story. For instance, an approximately 2- x 2- x 3-foot (60- x 60- x 90-cm) box may say that it can take care of 6000 gallons (23,000 liters) of water, but this would be true only if the pond contained just a few fish (certainly no more than 5). For this reason, when you’re looking for filters you should also ensure that you ask how many 20-inch (50-cm) fish the filter can support when they’re fed normally. By balancing the size of the pond with the volume of fish, you’ll ensure that you can sustain your koi with minimal risk of exposing them to harmful chemicals. Purchasing this filter system before populating the pond will ensure that, if checked regularly, your pond will be a sustainable habitat.

Therefore your filtration system and testing kits should be the first things you budget for when planning a pond. Pond supply retailers are the perfect place to go for quotes on filtration systems, testing kits, and other supplies, and they will help you factor in the overall cost for your pond before you start building. Always factor in all potential costs in your early planning stage so that you’re not caught by surprise later on.

There is no doubt whatsoever that koi are among the most graceful and attractive fish to keep. Because they live for around 15 to 20 years, they’re ideal companions that you can lovingly nurture. However, if you fail to look after them properly, they’ll never live to their full potential and will suffer. If koi are in your future plans, hopefully this guide has provided a good starting point for planning your pond and ensuring that your koi are given the best possible care.

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