Vertical pond walls are preferred in koi ponds because they increase the water volume and minimize the risk of a fish jumping out. This makes a block built pond a popular choice.

In this type of construction, the surfaces are rendered and sealed before use. For extra strength, incorporate glass fibers into the render mix and place reinforcing steel rods in the block work if the construction technique and design require it.

Some people choose to insert a liner into a concrete or rendered block pond rather than seal it. Apart from PVC or butyl rubber liners, both of which may crease, the choice is usually between MDPE or, more usually, glass reinforced plastic.

If you choose GRP, make sure that a competent person does the fiber glassing. The mix must be right to ensure a complete cure, as koi can accumulate potentially fatal styrene in the bodies from under cured resins used in the process.

Below the waterline pumped returns are useful features in the koi pond. They assist circulation, reduce surface disturbance when viewing the fish and prevent excess cooling of the water in colder months.

However, poor positioning of these returns will cause a break up of the circulatory flow and may lead to a build up of detritus in quiet spots. In ornamental ponds, it may be necessary to have several carefully placed returns to avoid this condition developing.

Some returns can include a venture device to improve pond aeration. Water pumped through a nozzle in the venture is fiercely aerated by a flow of air sucked in through a pipe leading above the surface.

To prevent koi from damaging themselves on it, make sure the venture does not protrude into the pond. You can position the venture outside the pond wall and fir an extension pipe that takes the aerated water through the wall and finished flush.

For the same reason, position waterfalls carefully so that they do not interfere with good circulatory flow in the pond.