One of the most important pieces of equipment when constructing a Koi pond has to be - a BOTTOM DRAIN. Without a bottom drain a Koi pond cannot function to it's supreme efficiency.
Bottom drains today are used in conjunction normally with a gravity fed filter systems, but more people are using them with pump fed systems. However, which ever way you decide, either way is far better than not having a drain installed at all. A chamber of filter brushes holding back debris in a pumped system cannot compare to a bottom drain working in conjunction with a vortex settlement.