Koi can grow as large as 3 feet and reach mature length in as little as 3 years. With good care and proper environment, they can live to be 60 years old. Koi are best suited to ponds or pools, rather than in-home aquariums. They are cold water fish, thriving best in temperatures between 61 and 75 degrees Fahrenheit. Water quality is the single most important factor in the health of your fish. Keep the number of fish in the pond relative to the number of gallons of water your pond holds, approximately 1 inch of fish per gallon of water.
For more information, please read the story Choosing Koi.