Koi can be kept in aquariums, but this isn't healthy for the fish if the aquarium isn't proportionate to their body size. Most adult koi live in outdoor ponds to accommodate their large size.
Koi commonly reach lengths of 2 feet or more, so most indoor aquariums simply do not have enough water volume to allow the fish room to move or breathe. Younger koi are about as large as common aquarium fish, but they grow rapidly and need to be placed in increasingly larger bodies of water. It is common for young koi to live in indoor aquariums until they reach an adult size. Fully grown koi are also kept in large aquariums or tanks when there is danger of the outdoor pond in which they live freezing completely. They are sometimes kept year-round in indoor ponds when the fish owner does not have enough space outside.