Koi Pond Basics
When designing the the pond you need to first choose your location. Under large trees is not idea since the falling leaves will be a constant problem. Also you will need to dig through dense roots. A spot that has partial shade and no runnoff from nearby sprinklers is ideal. Koi prefer the shade, but they do require some sunlight to maintian good health.
Once the location is decided you then need to then determine depth. Koi prefer at least 3 to 4 feet, but 6 or even 8 foot koi ponds are not uncommon. The deeper you go the better the koi will develop, the more stable the water temps will be and the more gallons you can have with a smaller footprint. With proper stocking density and filtration, you should be able to see all the way to the bottom of an 8' koi pond.
Once you have decided on the depth you need to consider quality of materials and budget. The most economical will be a EPDM liner pond. But these can leak over time and they also create folds which allow for debri to get trapped and decay. The other end of the spectrum is a cement pond made from either cinderblock or sprayed concrete. These are then sealed with a variety of products available that can be applied by the very brave do-it-yourselfer or a professional.