A pond measuring a depth of four to five feet is most favorable for your koi's health and survival for a number of reasons. In shallow ponds, the water temperature fluctuates more with climate changes. If the water gets too warm, the temperature can jeopardize your koi's health. Koi are a part of the carp family and are coldwater fish. It takes longer for temperatures to rise in deeper ponds with larger capacities of water. A deeper pond cultivates your koi's health and development by allowing them to exercise properly with longer swimming distances up and down. In an outdoor environment, a pond's depth is instrumental to your koi's survival. Three feet deep is the minimum height for protection against predators, i.e., raccoons, coyotes, cranes, and egrets.
Koi's prefer longer ponds to shorter round ponds. The pond's length is critical to the koi's health, promoting proper growth for the fish, and providing them with the appropriate space to exercise. A 6" to 8" long koi, can reach a moderate size of 2 feet long and upwards in length in 3 years, depending on the amount of food and how frequent you feed them.
The greater the volume (gallons of water) of your pond, the easier it is to keep the water temperature stable and healthy for your koi. Volume also applies to the stocking rate (number of fish per gallon of water). To promote a healthy environment, it is important not to overcrowd your pond with fish. Volume is the determining factor when deciding on the amount of fish you wish to keep.