Plumbing is one of the most important processes in building a Koi pond, and also the easiest to run into problems with, as there are so many variables. You want to make sure that you have all proper lengths and widths on pipes, appropriate pumps with the appropriate power, Filter(s), Water leveler, drains, etc.
One commonly overlooked specification is finding the correct sized pumps and piping. Originally I used a ___hp pump, and ___ diameter piping. I found that power wasn't distributed evenly throughout the Koi pond, so decided to use ___diameter piping, and 2 ___hp pumps. By doing this I was able to keep even pressure throughout all of the pipes, and in turn keeping happy fish. Also, having several low-pressure pumps keeps electricity costs lower. When determining what size pumps and pipes to use, you want to take into account how long your pipes run, the diameter of your piping, and the height in which water will have to be moved. Using the exact calculation is uncommon, but you can use simple calculations to figure out what you need to use.
Generally Schedule 40 PVC piping is common for most Koi ponds. Piping can vary depending on the circumstances. For higher pressure, you will want to use longer plumbing. Also, the higher the volume (waterfalls, etc.) you will want to use a larger diameter pipe.
During the planning process, it is good to not only review all building codes according to your city/ county/ state, but also national codes. Sometimes you will catch issues such as property lines and barriers, etc. I have heard stories of people using heaters for their Koi ponds, and placing them next to the property line, only to find out that the exhaust vent for the heater needs to be placed 4 ft. from the property line.
Once you have decided which equipment to use, it is time to install the skimmer and drains. The skimmer sucks up leaves and other dirt that collects on the surface of the Koi pond. The skimmer also collects water to the drains, so that it can be sent to the filter. The skimmer doesn't have a powerful suction, and only assists in sucking water to the filter. Most of the water that is fed to the filter is collected in main drains tied to the bottom of the Koi pond. To install the skimmer, it is best to place a piece of rebar or a metal pole above your desired location. From there, attach the skimmer with ties to the rebar so that it is in the exact place that you desire.
Next you will be installing the returns from the filter. Although there are two types of return systems, looped and non-looped, looped returns are the most commonly used. The advantage of looped returns is that the water pressure of each return is equal. The only advantage to a non-looped return system is that you save on PVC. The huge disadvantage to a non-looped return system is that the end returns have much more pressure than the returns closest to the pump.
When installing the drains, it is best to use two main drains that work off of one pipe. This system is called an Anti-Vortex drain. The reason for using an anti-vortex drain is for safety. If a fish was to get sucked against one drain, the other drain would end up sucking harder, giving the fish relief to swim away. This same system is done in recreational pools so that if someone is sucked against one drain, they wont be held down by constant pressure.
The next step is to install the equipment pad. I used prefabricated concrete slabs as my floor. I then placed my filter and pumps on the slaps once they were placed and leveled in my desired location.
Once the pumps and filter were in place, it was time to begin trenching. Usually a trench is dug between 6" and 8" deep, and wide enough to fit all pipes and lines. The trench should connect your Koi pond and Koi pond equipment. A connecting trench should then be dug around the perimeter of the pond with a width of 1'.
Once the trenching is complete, it is time to start assembling the valves and pipes. It is best to start with the valves closest to the equipment, but if you have others helping you, it is fine to start at both ends (one person at the equipment, and one person in the Koi pond). The pipes that were used for the drain are 2" in diameter. Before connecting the piping to the equipment, you want to give a pressure test. You do this by capping one end of the piping, and hooking up a pressure stack (a pressure gauge with a hose bib teed in) and filling the pipes with water with 35 – 40 psi. The pressure test is done to make sure that there are no leaks in the hoses. Since they will be under the 6 inches of fill, you want to be sure that all connections are tight, and there are no leaks before pouring fill. Generally the water pressure is between 15-20 psi, so knowing that it can handle 40 psi is a pretty good indicator that the pipes will be strong and tight. Once the pressure test is completed, you remove the caps from the ends, and hook them up to the skimmers.