How to Build a Koi Pond

Imagine sitting beside a pond gazing at the reflections of clouds on the water’s surface, when you become aware of movement and color within the water. Slowly, with regal silence, a group of large fish dressed in red, yellow, white, and black, loom upward toward you. And, just as slowly, they recede and move away.

It is moments like this that have made koi such an integral part of ponds for centuries. These fish have a presence in the water that must be experienced to be understood. The combination of size and color, along with the quiet grace of their movements, make koi a truly wonderful addition to a pond — adding to the attraction and charm that ponds already offer on their own.

A bit of Koi history

In China and Japan, where koi have been kept for many centuries and have been carefully bred for specific patterns of intense color or the absence of color, koi are passed down from one generation to the next. Koi commonly live to 70 years or more, and some have lived well beyond 100 years of age. It is no wonder that the keeping of koi in these countries is more than just an art and a science — it’s also part of the culture.

Koi Keeping 101

As with so many things, there’s more to koi and koi keeping than is immediately evident to the casual observer. The basics are simple but very important to know.

First of all, there are koi and there are koi. That is to say, the quality can vary tremendously, along with their price. Quality, in this case, refers to the intensity of colors and the patterns in which they appear.

Over hundreds of years, a variety of specific combinations of color and pattern have been selectively bred, and it is the process of culling out any fish that doesn’t meet these exacting requirements that ultimately accounts for the cost of the best quality koi. This is not to say you can’t enjoy koi that don’t meet such lofty standards, but rather, you should be aware that the koi found in most pet shops are simply not of the quality you would find in the ponds of koi breeders.

The experienced eye of the koi breeder looks for more than colors and patterns in the young fish that conform as closely as possible to the standards. It’s also important that these colors and patterns remain stable as the fish grows, which means breeding top-quality fish that already exhibit the best qualities in order to have the best possible gene pool to start with.

You can easily pay hundreds, even thousands of dollars for the best quality koi, but you probably don’t want to do this even if you can afford to. After all, until you have some experience keeping koi you always run the risk of the fish becoming ill and dying, or jumping out of the pond and dying, or being captured by a predator. Koi are hardy, but pond keepers new to koi can sometimes make mistakes the fish won’t survive.

The secret is to buy koi of fairly good quality that are quite small. The prices for these fish can be as little as $30 to $50. You do take some risk that the colors and patterns may change over time as the fish grows, but the odds are more in your favor with these better quality koi. This means buying koi from a breeder, a specialty water garden retailer, or even by mail from a reputable dealer — keeping in mind that air freight is costly.

When buying koi for a pond, the saying “too much of a good thing” takes on new meaning. Overcrowding a pond with koi accounts for perhaps 90 percent of all health problems with the fish. No amount of filtration can prevent the inevitable decline in water quality that will occur when there are simply too many fish in too little water with insufficient surface area. Filtration only slows down the process. The use of filtration allows increased stocking densities, but relying on filtration comes at a price — literally.

Overcrowding a pond with koi accounts for perhaps 90 percent of all health problems with the fish.

If the power goes out on a hot summer day, the fish in an overcrowded pond are all but doomed, because even the most sophisticated, expensive filtration system requires electricity. Hundreds and, yes, even thousands of dollars of fish can be lost. Sometimes this happens quickly, but often the fish react to the stress by becoming sick and dying weeks later.

Wise koi keepers even remove some fish from their ponds as the koi grow in order to maintain the appropriate biological balance in the pond, although this would be unnecessary if the pond were stocked with only enough fish so that the pond would not be overcrowded when the fish reaches adult size. And if the fish breed, the same process of removing some of them is also necessary.

Large water changes can help maintain good water quality, but may not be practical. In many areas, this is expensive at best, and often impossible if there are seasonal water shortages.

Koi grow large. Adults can be 30 to 36 inches in length, and sometimes even longer than this. More importantly, koi have a lot of body mass as well, a fact not sufficiently appreciated by enough koi keepers.

How many koi per gallon?

Mass, or weight, represents the biological fish load in a pond. When the total mass of the fish exceeds the natural carrying capacity of the pond, problems with water quality are inevitable. Some stocking guidelines concentrate on the ratio of the number and size of fish to the volume of water, but the ratio of fish mass to the total surface area of the pond is just as important.

A pond’s surface area matters when determining stocking densities because most of the dissolved oxygen in the water is acquired directly from the atmosphere at the pond’s surface. And because koi are large fish, they need a lot of dissolved oxygen in the water.

A realistic guideline for stocking a pond that doesn’t have supplemental biological filtration is 12 inches of fish for every 100 square feet of surface area, assuming the pond is 2 feet deep, or more. Although only a guideline, it emphasizes just how few fish can be kept safely (meaning disease-free) in a modest size backyard pond.

Remember, adult koi can grow to 36 inches given optimal conditions. A pond with a surface area of 2000 square feet (a 10- by 20-foot pond), for example, would safely hold six or seven full-size koi. Adding a biofilter would allow more fish, but you run the risk of problems should the pump stop for more than a few hours.

And the costs of running a pump for a filtration system are greater than you might imagine. You may find your electric bill going up by $50 to $75 a month just for this.

When it comes to stocking a pond, the “less is more” principle is a valuable concept to keep in mind. That is, a small number of carefully selected koi can actually be more enjoyable than having many. In fact, you will find that you pay more attention to each fish’s qualities when there aren’t as many competing for your attention.

There is an important added benefit to paying attention to the fish that goes beyond admiring their beauty and grace. Careful observation is how you notice if there are any problems that might otherwise go unnoticed.

Although disease and parasite problems are relatively rare when a pond is stocked correctly, if there is something wrong, you are more likely to see it when there are not as many fish. Noticing any changes in a koi’s physical appearance and behavior is often the first sign of a problem, but to notice changes, you first have to be aware of the normal appearance and behavior of the fish.

Being able to see the fish easily, however, is not always as simple as you might think. The design of the pond can have a significant effect on how difficult or easy the fish is to observe. And the clarity of the water also affects the visibility of the fish.

Pond Design

In terms of pond design, a large circular pond with a deep center is the worst choice when it comes to being able to see the fish. The koi are simply too far away to observe because they tend to stay near the center, often nowhere near the surface. An irregular pond shape that keeps the deepest areas of the pond within a few feet of the edge will make a huge difference when it comes to seeing the fish.

You may want to have benches or a deck with chairs adjacent to the pond so you and your visitors can relax and enjoy the fish, as well as the pond itself. The more inviting it is to spend time next to the pond, the more likely you are to do so.

Water clarity varies with the season and the amount of organic matter in the water. Many ponds have periods of green water as unicellular algae proliferate in the spring — when warming water temperatures, increasing sunlight and rising nutrient levels in the water come together to create a perfect environment for the algae. If the pond is not overstocked, if the fish are not overfed, and if other plants and/or types of algae consume most of the nutrients, the clarity of the water will improve over a number of weeks until it is possible to see the bottom of the pond.

Not that the water will be crystal clear. Dissolved organics tint the water to varying degrees during the course of the year. A water change will temporarily reduce the concentration of dissolved organics and make the water clearer.

One other aspect of a well-designed koi pond is some type of water feature that helps aerate the water while providing an aesthetic enhancement to the pond. A waterfall that has numerous small steps will raise the dissolved oxygen content of the pond water while providing a wonderful visual and audible treat.

In the summer, when the water is warmest, it holds much less dissolved oxygen than it does when it’s cooler. Although a properly stocked pond of a given surface area is less likely to have insufficient dissolved oxygen for the fish, this can become a problem if there’s also a lot of algae and plants in the pond. During the night these plants take in large amounts of oxygen, and by early morning the fish may have to remain at the surface of the water to obtain adequate amounts of oxygen. During the day, these same plants release oxygen back into the water during photosynthesis, and dissolved oxygen levels return to normal.

Chronically low levels of oxygen will stress the fish, and once again the prospect of disease increases. If a waterfall isn’t practical or desired, a fountain can be used for the same purpose. Or, a wet/dry (trickle) biofilter can help raise oxygen levels in the water.

Koi ponds are not the same as garden ponds. The presence of these large fish completely changes the biological balance of the pond in ways that affect the fish themselves.

Maintaining the appropriate water quality and chemistry takes on greater importance in a koi pond. And this becomes increasingly difficult to accomplish as the fish grow in size. Modest stocking levels are crucial.

Finally, a small, intimate garden pond cannot be a suitable home for koi. A koi pond has to be spacious, with lots of surface area. Garden ponds have mostly plants, koi ponds have mostly fish. You have to make the choice.

Garden and koi ponds are both nice. Before you make your choice, you might want to reread the opening paragraph of this article.


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