Koi pond construction requires a little more planning than other kinds of ponds, to make sure you've met all your koi's needs. Here are some tips to help you build your koi pond.
Planning your koi pond construction
First of all, think about a site. See koi pond design for more on this – there are a few things to consider about location before constructing your koi pond.
Next, think about how many koi you want to have in your pond – the more you have, the larger a pond you'll need. (More about koi pond size and depth.) The best piece of advice I can give here is that it's better to risk constructing your pond too big than too small. It's very difficult and expensive to make a pond bigger once it's been built; and the odds are that once you start buying koi, you'll want to buy more! (Buying koi can be kind of addictive.) An awful lot of koi enthusiasts find they wish they'd built their pond bigger.
Choose your method of filtration before constructing your koi pond, as it will affect the materials you use, and you may need to build drains into the bottom. You'll also need to plan space for your filter system. Koi are very large fish, eating a lot of food, and their water fills up with waste quickly; that means that koi ponds need much larger filtration systems than most ponds.
Construct your koi pond with slightly raised edges (raised above ground level). This will stop rainwater running into the pond from your lawn – plant fertilizers and weedkillers will do very bad things to your pond water.
It's a good idea to construct your koi pond with possible predators in mind. Koi's bright colouring makes them very visible, especially from the air. To stop herons fishing your koi out of the pond, make sure that at least part of your koi pond is too deep for them to stand in. To stop foxes and other mammals, it's a good idea to make the sides of your koi pond steep or overhanging, and high enough above the water level that they can't reach in.
When to build your koi pond
The best time for koi pond construction is summer or early autumn. For one thing, you get more hours of daylight to build in. For another, in the winter or spring the ground is likely to be either harder or wetter, which makes it harder to dig.
Methods of koi pond construction
Building your koi pond with a liner
If you're creating a fairly small koi pond, you could construct it with a pond liner – butyl rubber, or pvc. Make sure you get a high-quality liner, at least a millimetre thick so that it won't tear, and of a material that won't become brittle over time.
Do also make sure your liner is large enough for your koi pond. To calculate the length of your liner, take the depth of your koi pond and multiply it by 2, then add the the length of your pond. Do the same for the width. Then add a foot all the way round (so two feet in total to each measurement) for overlap around the edges (where you'll lay stones or paving slabs over the liner).
Using a liner of this kind is the least expensive way to build your pond, but it's not the most attractive option. Because koi ponds tend to have fewer plants than other ponds, the folds of the liner may be quite visible. Don't be tempted to cover them with lots of rocks and pebbles – this just results in a build up of waste at the bottom of the pond, which makes for very unhealthy water for the koi.
Check for sharp stones before laying your liner, as these can pierce it. It's a good idea to place a layer of sand – about 10cm – at the bottom of your koi pond, beneath the liner, to protect it from sharp stones or sticks in the soil, and you can protect the sides by putting underlay under the liner (use a synthetic one so it won't rot).
If you want to build a bottom drain (see koi pond filtration) into your koi pond, you'll need to put it in a bed of concrete.
Don't secure the liner before filling the pond. It will probably stretch under the weight of the water. So fill it gradually, and adjust it as necessary as you go – only secure the edges of the liner once the pond is full, and similarly, don't cut holes for piping etc until the water is just below the relevant point.
Building your koi pond with concrete
Another option for koi pond construction is concrete. This is more expensive, but avoids the unsightly folds that you get with a rubber pond liner. It also allows you to make any shape you want. The smooth bottom is also healthier for the koi, as they are bottom feeders.
A concrete pond works really well with a bottom drain to pass debris to the pond filter (about one drain per 10 feet). You'll need to put all piping in place before you start laying concrete.
If you're using concrete to construct your koi pond, you won't be able to have completely vertical sides, as the concrete will just pour off – a bit of a slope will work better. Wire mesh will help stop the concrete sliding while it's wet. You'll need about a foot of concrete at the bottom, and about half that at the top of the sides.
Concrete leaks cement into the water, which raises the pH (koi need a fairly low pH). So the surface will need to be treated. Let the initial concrete layer dry for three days, then render it with a mixture of soft sand, cement, and a waterproofing agent – a little over an inch thick. When that's dry, cover the entire surface with a waterproof sealer. Give it a second coat 48 hours later. Wait four days before you fill the pond, and then at least another two before you try introducing your koi – but do check the pH first, to be on the safe side.
Building your koi pond with fibreglass
Fibreglass is tough, and like concrete, can be used in a pond of any shape. It's a good idea to apply the fibreglass to a layer of cement.
Building your koi pond with bricks
Bricks or breeze blocks are another option – you just build the shape you want, then apply a waterproof sealer. An advantage of this method is that you can get a true vertical for the sides, if you want. A dis advantage is that bricks lend themselves most naturally to straight walls and sharp corners, so this isn't a great method for Japanese koi pond construction, as the Japanese aesthetic prefers natural curves.
Building your koi pond with a prefabricated pond
You can get prefabricated fibreglass and plastic ponds in all kinds of shapes. They can make koi pond construction a lot simpler.
The downside of this method is that it isn't so good for large ponds – the hassle in getting a really large prefab pond delivered, let alone getting it into your garden over fences or through gates, not to mention the weight, is not to be underestimated. And koi really do need a lot of space and depth, so don't be tempted to get a smaller prefab just because it's easier if it's not big enough for your koi.
After you've dug the hole for your prefab pond, use a layer of soft sand to settle the prefab in, and protect it from sharp stones which may puncture it once the weight of the water's been added. The sand will also make it easier to fidget the prefab until it's level.
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