Koi Pond Construction Basics

Section 1 – Choosing the location

One of the biggest challenges in constructing a Koi pond is finding the right location for it.

Having a koi pond that is either inaccessible or hidden defeats to the reason why you installed a Koi pond in the first place!

Here are some points you should consider when planning your koi pond installation:

1.)  Accessibility – you wouldn’t want your pond to be inaccessible and away from people. One of the great things about keeping a koi pond is it immediately brings delight to the people who see it. Having to go through several obstacles just to see your fish isn’t going to be convenient to guests that want to check out your pond.

2.) Visibility – the view of your pond should be largely unobstructed. One thing many folks do is plan for a dedicated viewing area.

3.) Cohesiveness – your pond should blend in perfectly with its surroundings, not stand out or be swallowed up by it. Your pond should look like part of the landscape. As a word of caution, be aware that shade is good for your pond and fish but putting your pond near/under trees will result in falling leaves into your pond that will decay and cause water quality issues.

One way to effectively visualize your pond for appropriate fit into your surroundings is to mark out the spot where you’re going to put it with spray paint. A bright orange spray should make it stand out so you can view it from different angles. Other folks will lay down string or garden hose to map out the pond.

Once you’re satisfied with the location of your pond it is the right time to start digging. Give it a couple of days or so before you pick up the shovel just to make sure you are 100% happy with the location.

Section 2 – Size and depth matter

Once you’ve decided where to build your pond, the next thing you need to do is determine how deep it will be.

Not surprisingly, you will need a larger pond for a greater number of fish but you will also want to plan for the increasing size of fish through the years.

For the average koi pond keeper, a pond 24 feet long by 12 feet wide with a depth of 4 feet should be sufficient for mid-sized koi. This depth will also allow for pond freezing in winter. This volume should be enough to accommodate ten 24 inch fish or twenty 12 inch fish without overcrowding issues. Another good rule of thumb for both aquarists and pond keepers is “less is more”.  More fish in a pond won’t necessarily bring you more happiness. In fact, more fish can sometimes translate into more maintenance and issues.

In order to calculate the volume of the pond you are wanting to build, use this formula:

Length (of the pond) x Width (of the pond) x Depth x 7.5 = Volume of water (in gallons)

or see this online calculator.

Section 3 – Everything falls to the bottom

Since your new koi pond is occupying a large space on your property, things are eventually going to fall into it. Leaves, dirt and other debris are going to find their way into your pond. And that doesn’t take into account all the fish food and droppings that are eventually going to find their way to the bottom of the pond.

This is where the bottom drain comes in.

Bottom drains in koi ponds are pretty standard and a four inch pipe leading out is optimal. Organic matter that falls to the bottom will eventually become pond muck. It will build up and make your pond stink which defeats the purpose of having a koi pond.

A bottom drain effectively sucks this all up, sends it into the filtration system for proper filtering, and disposes of it. The bottom drain is your first line of defense for muck build up.   The “old school” mode of thinking was to simply have a gravel lined pond bottom which, on the one hand is good because it allows for lots of surface area for beneficial (nitrifying) bacteria but on the other hand it allowed lots of muck to accumulate.   This resulted in the necessity for regular yearly (usually spring time) cleanings of the pond which involved draining the pond and removing all that slop. Very messy! The bottom drain will drastically reduce the amount of muck removal and cleaning you will have to do.

bottom-drain for koi pond

Typical bottom drain

Tip: Be sure that your pond is not too close to trees as leaves are bound to end up in your pond and will not only be unsightly but can decay and throw off the chemical balance of your pond.

Section 4 – The Filter System

Achieving clean water is ensuring good koi health. For a koi, the pond is their entire world. That means they breathe in it, sleep in it, feed in it and eventually pass their wastes in. If you don’t have a pond equipped to handle the wastes, it could become very dirty in a short period of time and you’re going to end up with very sick and unhappy fish.

You need a proper filtration system to avoid this.

There are four essential components of a koi pond filtration system.

Bottom drain – we’ve already covered this in the previous section so you are already familiar with the function of this and why it is essential to your pond’s success. A 6000 gallon pond can function properly with just one bottom drain. A larger pond will mean installing another bottom drain to effectively clean your pond’s bottom (benthic) surface.

bottom drain and cross section

Settling chambers – the bottom drain typically dumps out at the settling chamber.  As the name implies, this is where heavier solids settle out and the pump then pulls water out of the top of the settling chamber, which then passes through mechanical and biological filtration and back to the pond.

settlement chamber

Settlement chamber

Mechanical filtration equipment – this is what filters out the bulk of the debris. This happens as particles are trapped in the brushes of the equipment. Since this will eventually get clogged you must make sure you buy the equipment that ensures you will always have replacement parts or brushes when it is time to replace them.   Mechanical filtration is usually set up in stages where coarse mesh material catches larger debris first then subsequent filters are finer mesh and sieve out smaller debris.

Biological filtration – this is the chemical portion of the filtration process. Ammonia and nitrites are ever-present in bodies of water where fish live. These are the products of their waste which need to be processed in order to make the water habitable. Your pond performs this biological filtration on its own to a certain degree but having dedicated filter media where beneficial bacteria can thrive and perform the important work of assimilating the harmful ammonia, nitrites and nitrates is crucial to the longevity of your pond.

As soon as everything is broken down, the processed water will go back into your pond, much healthier than it started. Testing for ammonia and nitrites should also be done on a regular basis to ensure that everything is functioning well and that your biological filtration is in good shape.

Tip: Some pond owners are confounded by sudden increases in nutrients -this can often happen after a rain storm to ponds that are near places like golf courses or farmland.

book on how to build koi pond

Got questions? This book is a great reference to have around both for planning a koi pond and while you’re still waist-deep in a hole trying to figure things out.

Section 5 – Time to bring out the shovels

 There are three primary ways to install a pond.

• Prefabricated mold (plastic or fiberglass)

This is the most expensive option but what you save is hassle. This system only requires that a hole be dug that matches the mold’s shape.

• Liner (vinyl or butyl)

This is probably the most cost efficient manner of creating a pond and probably the most popular method used by koi owners all over the world. It allows for a lot of customizing and you can really use your imagination with this material. What you save in cost is countered by the greater effort to install.

• Concrete

This is the most expensive manner of fabricating a pond but also the one that provides a lot of flexibility in terms of creating a pond matching your expectations. Though more expensive than the previously-mentioned methods it offers substantial strength and leak-proofing.   Another added benefit is that is helps to buffer swings in pH due to the carbonate in concrete and grout.

It helps to set out all of your materials so you know where your equipment is going to go when the pond is complete. Measure each piece so you know the actual dimensions and determine just how far you should dig. In the following example a pond is being built using a vinyl liner.

Excavate the entire area within your outline to about 9 inches deep. This should effectively set your pool area apart from the rest of the area and give you a better visualization of what it will look like in the end. Avoiding a perfect circle shape for your pond will help steer clear of the “necklace effect” which results in a “necklace” of rocks lining the pond edge. A better shape to shoot for is essentially an irregular shape that allows your pond to look more natural.

The next thing you should do is dig a trench along the outline you traced on the ground. A trench about 12 inches wide and 12 inches deep should be sufficient. This is going to be your initial shelf where your ornamental rocks will be positioned.

Place flat wooden stakes all around the diameter of the trench’s sides. This means both sides so you’ll have wooden stakes on the outer layer and the inner layer. This is where you are going to place your plastic shaper so that the cement you pour in is going to conform to the shape of the trench.

Before pouring the cement in, place some rebar to create a basic framework or support system for the concrete once it hardens. This will also allow the cement to retain its shape once this process is done. Tie the ends of the rebar together with some wire and then elevate them with a small block of cement as evenly as you can all throughout the diameter of the trench.

Now pour the cement in. Make sure that the volume of cement per feet inside the trench is consistent. After that, use a wooden planer to level out the top of your cement.

Allow for the cement to harden before moving on to the next step which is to remove the plastic shapers and continue digging out the hole. The cement should be ready after letting it sit for about 24 hours.

After that process is done, you can excavate the area interior to the concrete. With the cement bench in place, you should be able to excavate as vertically straight as possible along the edges.

With respect to depth, a good depth should be around 4 to 5 feet. It’s deep enough for the fish to have some variety in terms of swimming depths, thermal refuge during winter and shallow enough for the average person to get into to provide preventive maintenance care on a regular basis.

Make sure the bottom of your pool has a gradual slope towards the center of your pond. This is where you’ll be installing your bottom drain. Another thing to note is that if you live in an area cold enough to cause your pond to freeze over you will want to angle the sides of your pond in (towards the pond center) around 15 degrees to allow for the expansion of ice to slide along the angle and not outward against the liner.

The next step is install the bottom drain and anchor it at the lowest part of your pond. You will need to dig a deeper hole to set the foundation of the bottom drain and a trench for its pipes to run through. And then you will have to anchor it down so there is no unwanted movement which could compromise the fittings of the pipes. This movement is usually caused by water pressure from all the water movement.

The next thing you need to make sure of is that you have the proper returns and fittings installed for the pump and filtration system. One great thing about the filtration system is that you can elevate it and hide it with some strategically placed rocks to emulate a waterfall. This should help you create a waterfall effect. Your returns on the other hand can be placed in several locations to create several vortices to direct the water flow.

Another important addition for your pond is a skimmer to keep the surface of your water looking great and to keep floating debris like leaves from ever making it to the bottom to decay.

skimmer box

Skimmer Box

Whether you have a sunken pond or a raised fish enclosure, it’s always best to keep your liner and plumbing hidden from view. Nothing is more unsightly than exposed plumbing and liners. But there’s another reason why you should hide these from view. Since your pond is essentially going to be an outdoor fixture, it is going to be exposed to the elements. Your liner and plumbing which is made of a plastic and vinyl mixture is eventually going to be damaged over time and either crack or warp.

Remember the shelf you made with the cement? This is where your liner will rest being held in place with rocks over the edges. Do not cut the edges of your liner until you are absolutely sure that everything is set and in place. You will want to lay down a layer of sand or something that will help cushion the liner from possible puncture prior to setting in the liner.

Another thing to be aware of is to ensure that the edge of your pond is slightly higher than the surrounding areas to prevent rainwater from flowing into the pond. An elevation of an inch or two should suffice unless you live in an area that gets plenty of rainfall. 

Basic Pond Construction with Liner

Final Thoughts

The methods and techniques presented here are typical and used successfully by many however it should be noted that there are LOTS of ways to build a healthy pond. There’s lots of ways to filter water and not just one, tried and true method. For example, some folks don’t have bottom drains, some folks would never consider using a liner while others swear by it. Your particular situation and finances will dictate what kind of pond you install and the size of your filtration you need.   Beyond the foundation of your pond, the components you want to put the most energy and investment into is your pump and filtration because those are your heart and kidney’s of the pond.