Koi, or carp, are colourful ornamental pond fish that originated in Europe but have been raised by hobbyists in Japan and elsewhere for hundreds of years. The fish grow within a couple of years to a fairly large and substantial size -- and they can live to be 70 to 80 years old -- so it is important to calculate how much pond you will need for the fish you intend to acquire.
Koi are fairly hardy and will tolerate less than sparkling water but do need some depth in a pond to overwinter. A pond that will house koi throughout the winter should be at least 3 feet deep in its deepest section. Depths between 3 and 4 feet are recommended by Utah State University Extension, particularly for koi ponds in Northern regions. A pond that deep will not freeze solid and the koi will hibernate in the depths of the pond.
Jade Dragon, a hobbyists' online resource, says that koi are communal fish and like company. The site recommends keeping at least three fish and says you need 50 to 60 gallons of water in a pond for each 8- to 10-inch koi. But it's important to remember that koi grow, and in two to three years, that nice little fish could be 2 feet long. There are reports of Koi reaching up to 6 feet but more common mature sizes are in the 2- to 3-foot range with weight around 9.07 Kilogram.
Fish Per Pond
The Southern Regional Aquaculture Center offers this example of how to calculate appropriate balance of koi to pond. For an unaerated pond, you need 4.5 square surface feet per 12-inch fish. If your pond is 9 by 15, surface square footage equals 135. Divide by 4.5 to determine there is room for 30 foot-long fish. But koi are bigger than that. At 18 inches average per fish, for instance, that pond could handle only 20 fish. Aeration from a pond waterfall allows you greater density -- a 12-inch fish for every 2 or 3 square feet of pond surface. Serious hobbyists, with valuable fish, stock no more than 10 to 15 fish per pond or tank.
Gallons Per Fish
If you want a small water garden, forget koi. A larger pond is a more stable aquaculture and they are messy fish in need of good filtration and lots of water. The University of Rhode Island GreenShare says koi ponds should begin at 500 gallons of water and that most ponds with koi average about 1,000 gallons. Figure number of gallons this way: length of pond in inches times width in inches times depth, multiplied by .004329. Provide about 250 gallons or more for every 8 inches of fish, excluding tail fins.