Koi can live in aquariums until they are six inches in length, or about two years old. The most important considerations in tanking koi are the number of fish in the space and water quality conditions.
A 55-gallon aquarium works for smaller fish, but a 125-gallon aquarium is ideal. The larger tank comfortably houses nine six-inch koi. Koi never stop growing, so it is important for koi owners to adjust the number of koi housed at any one time. A high-capacity aeration unit is essential to maintain air movement in the smaller space, and a dissolved oxygen level of seven parts per million or more is recommended.