Mains powered air-cooled external pumps are ideal for handling large volumes of water, so are well suited to a Koi pond. However, they do have higher running costs than submersibles.

External pumps must be operated in cool, dry surroundings, such as an outbuilding or an insulated, weatherproof but well ventilated chamber. For unobtrusiveness and temperature control you could dig a pit in the ground, but it's also possible to disguise an above-ground chamber using trellis, decking, rockery or whatever else is appropriate to the pond's setting.

Many fish keepers, especially Koi keepers, choose to use industrial-grade central-heating pumps because of their silent operation, reliability, and relatively low cost. Despite this, manufacturers and suppliers do not encourage this method. If you do choose this option, it is recommended that you install an electronic float switch stepped down to 12 volts to avoid electrocuting the fish if a short circuit occurs. The float switch will protect the pump by switching it off if the pond water levels drop dramatically as a result of leakage or evaporation.

External pumps need filling with water before use (a method known as priming), but if the pump is below the pond water level, gravity will do this for you. Unless it's a self-priming type you'll need to prime once that's sited above the water level by using a footvalve on the inlet tube. And even self-priming pumps need you to fill them with water initially.