The standard rule is that a pond should have 10 gallons of water for every 1 inch of fish. The average length of a mature goldfish can range from 8 inches to 10 inches and some breeds are even longer. The average size of a mature koi is 12 inches. The “gallon to inch” ratio allows fish to thrive, reach their maximum size, and helps keep ammonia levels manageable, which is the leading cause of fish deaths. It becomes extremely difficult to maintain good water quality when the pond is overstocked.