Design a Koi Pond
A good koi pool will have as large a surface area as your plans will allow. However, it must have adequate depth of at least 4'6" and will have a bottom drain, with or without airdome (the choice is yours), a proper sized filter system, U/V, airpump and surface skimmer, multiples of these on a large pond. The pond would be built out of concrete blocks, cement rendered and waterproofed. (See category 'pond waterproofing'.)
