Koi fish thrive in large outdoor ponds of at least 1,000 gallons, with a depth of 3 feet or more to accommodate their growth and seasonal temperature changes. They prefer water temperatures between 65-75°F but can tolerate colder conditions. A strong filtration system is essential to maintain water quality, along with regular water changes (10-20% weekly). Koi require high oxygen levels, so aeration through waterfalls or air pumps is beneficial. They are social fish and can live peacefully with other koi or goldfish. Feed them high-quality koi pellets, supplemented with fruits and vegetables. Test water regularly for ammonia, nitrites, and pH balance, keeping it between 7.0-8.5. Managing algae and beneficial bacteria helps maintain a healthy ecosystem. Koi can live for decades with proper care, providing beauty and serenity to any pond.