Building a koi pond is very different from a nature or wildlife pond. Japanese koi fish limit the amount of plant life you can grow in the water, simply because they eat greenery. A koi pond needs to be a good size, since the fish can grow really large and impressive. As a rule you need at least 1000 gallons of water, but the more the better. And it should be at least 3 feet deep, ideally nearer five feet, especially if you live somewhere where the surface of the water can freeze solid in winter.
First, choose your location. You’ll get more out of your pond all year round if you place it within view of the house, since ponds of all types tend to attract interesting and sometimes rare wildlife. Make sure to position the pond where rainfall run-off won’t flow into it, together with the fertilizers, chemicals, and organic debris it can carry.
Don’t make a pond too near trees. You’ll only have to spend time dredging smelly debris and leaves out of the water. Ideally a good koi fish pond needs at least 6 hours of direct sunshine if you want to grow water lilies in it, something a lot of koi keepers enjoy because the flowers are so beautiful and the leaves provide important shade for the fish.
You don’t have to buy a water pump. Your koi will be fine without it. But it really does help because it lets you keep more fish – simply because there’s more oxygen in the water – and it also helps water plants thrive. Plus, the sound of running water is one of the loveliest sounds in a garden, relaxing and peaceful.
Most ponds benefit from a biological koi pond filter, but it’s totally essential for koi because they are big animals and generate a lot of waste in the shape of fish poop, which soon contaiminates the water. Even a small school of goldfish will soon mess up their own water.
How do you choose the right size for your koi pond? It’s often tricky to do it in your imagination. It’s far better to go outdoors and use a ball of string to demark the shape and size. It helps to know that the biggest mistake most people make is building the pond too small. The bigger the pond, the more ecologically stable, easier to maintain in good condition, and more pleasurable for your fish to live in.
After you’ve laid out the shape, measure the actual length and width. Add the depth twice, plus two feet of overlap, and you’ll have the right amount of pond liner. Measure twice and buy once – there’s nothing worse than a liner too small for the hole!
If you intend to fill up with water plants, create a shelf around the edge of the pond to stand the plant containters on. A foot deep and a foot wide should do the trick. Then dig the rest of the hole with a slight slope at the end to let wildlife crawl out to safety if any falls in. Don’t forget to dig a proper ditch for the wires and plumbing, stuff like your pond filter, pond skimmer, waterfall or whatever.
Line the pond with the best quality butile pond liner you can afford. The best have a 10-20 year guarantee. You can cut it with scissors or a Stanley knife. Once it’s the right size and shape, lie it carefully in your hole. Some people first line the hole with old blankets and duvets, as an extra layer of safety against tiny sharp bits of stone and sand which can result in damage to the liner. Do your best to avoid folds and wrinkles, only leaving small ones that will flatten out naturally when you add the water.
This is the point at which you start creating cool extras like waterfalls and streams, excavating the space for them so they fit with and complement the pond properly. If you’re creating a stream, dig it wider than the finished size to make room for stone edging. You can glue down the stone using mortar or expandable foam. Arrange the stone, other coping or grass turves around the edge of the pond, folding the pond liner up behind the stone or turf so it sits just above the water level. If the stones you’re using are big and stable, you can do without mortar. Back fill the gaps with soil to hold the pond liner against the stone and hide the liner from view.
Finally, fill the pond with water up to a few inches from the top. Then you simply wait for the water to go green, then clear. Once the water has ‘sweetened’ – no longer green and with no harmful chemicals left over from the tap water – and everything has settled in properly, you can add your plants and fish. If you don’t want to wait for the water to sweeten naturally, you can use a special dechlorinator product to get rid of chlorine and chloramines. You might want to add fish gradually over a few weeks rather than all at once.
Some of the best plants for koi ponds include Anacharis, Water Hyacinth, Parrot’s Feather, Water Lilies and Bacopa. Use their leaves to shade round two thirds of the surface area if the pond is in full sunhine. If there’s less sunshine, you can use fewer plants and shade less of the surface.
A quick word about koi pond kits. If you like you can buy a koi pond kit including wall the equipment and materials you need, saving you money compared to buying the bits you need – things like water garden liners and fish pond pumps – individually.