Koi

First things first, calculate what size pump you need. Work out the gallonage of your pond: length (ft) x width (ft) x depth (ft) x 6.23 = total gallonage. Your pump should be capable of pushing half the volume of your pond through the filters once every hour. A 2,000 gallon pond, for example, will need a 1,000gph pump. Check the flow rate specified by your filter before taking the plunge – bead filters and media showers function at a very high flow rate.

It requires more power to push water through narrow pipes, so always opt for the largest diameter pipe that your chosen pump supports. Items like venturi, UV units and heaters will all affect the flow rate too, so you may have to up the pump’s power if you’re planning on running a lot of equipment.

Where you plan on positioning your filters will also determine what size pump you need. It takes more energy to pump water to filters housed 2ft above water level, say, than to filters housed at the water level. The height to which you want to pump water is known as the head, and most pump manufacturers include a handy graph on the packaging that explains how the head influences the flow rate.

Sump pump
Pros:
• Submergible
• Moves large volumes at high pressure
• Delivers good flow at head height
• Cheap purchase price
• Useful for pumping to waste

Cons:
• High-wattage; expensive to run
• Limited 12 to 24-month guarantee

Cost: £60 to £70 for a 1,500gph model

External/submersible pump
Pros:
• Can be used in or out of water
• No need for waterproof housing
• Low wattage in relation to flow rates
• Ideal for typical gravity-fed set-up
• Long guarantee period – up to five years with some models
• Low running costs – will pay for themselves in a year or two

Cons:
• Older models’ strainers get clogged quickly
• Low wattage means limited flow pressure
• Need to up the size if pumping to head height

Cost: £300 to £350 for a good 16,000lph model with five-year warranty

External pump
Pros:
• High-wattage high-pressure models deliver high flow-rates over long distances
• Manufacturers offer a full-service facility

Cons:
• Low-wattage models lose flow if pumping to height or over distance
• Not designed to be exposed to water
• Short guarantee period (usually one year)

Cost: £250 to £350, but you really do get what you pay for

Written by Keith Holmes