Breeders offer show and pond koi.
image by Public Domain, DRW & Associates, Inc
Homeowners who've never had a garden pond sometimes dream of lovely koi drifting languidly through fields of water lilies. Those who've kept koi know that these colorful carp are demanding guests that can sometimes grow 18 to 20 inches long and live for decades. Building a koi pond is not just a "water feature" project; it is a specialized venture, requiring knowledge and considerable planning. Keep in mind that you will be building a habitat for special---and rather expensive---creatures as well as a garden element.
Excavate your pond; unless you know your way around power equipment, it's wise to have this done by professionals. Site the pond in an area near the house that is easy for you to check daily. Stay away from trees to avoid tree sap and bird droppings in the pond. Make it at least two and a half times as long as it is wide and at least three feet deep. In areas where winters are cold, plan your depth to be at least five feet. Sides should slope down to a center bottom, and additional drains should be added every 10 feet or so around the deepest drain. Your pond will need a volume of at least 100 gallons per fish. (Remember; you're stocking it with fish that may grow over two feet in length.)
Dig a settlement and filtration pond, one-quarter to one-third the area of the main pond. Make this pond three feet deep with a sump pump or drain pipe to evacuate (or "backwash") the bottom of the pond where the muck collects on a daily basis. Hook up the biological filter to draw water from the upper part of this pond and cycle filtered water back into the main pond over a waterfall. The filter should be able to cycle the volume of water in the main pond every one to two hours. You'll need enough PVC pipe to connect the bottom and side drains and a pump to move the water from the main pond.
Install an overflow pipe about six inches below the surface and run it out into the garden. Add skimmers---either the wide, flat inlets sold to install around the edges of swimming pools or a plain four-inch PVC "standpipe"---to collect leaves and other debris on the water's surface.
Add oxygen to the water with an aerator, waterfall, water jets or some combination of these. Waterfalls provide a turbulence that koi enjoy. Koi are carp---stream fish whose natural habitat is running water---so move water in a counterclockwise pattern with your oxygenation and filtration systems. Add bottom water jets to ponds deeper than four feet and put valves on the supply pipes so the water jets can be adjusted and turned off during the winter when fish are inactive.
Supply the pond with fresh water. Koi will need 15 to 25 percent fresh water each day to replace the water lost in backwashing the settlement pond. Water from a city water supply that uses chlorine should be trickled over the waterfall to allow the chemical to evaporate. Put a valve on the supply pipe to control flow.
Consider optional equipment like an ultraviolet (UV) light sterilizer for the filter that kills parasites and improves water clarity. Pond heaters can protect water from prolonging fluctuations through this zone as the seasons change.