A 10-gallon tank is unsuitable for keeping any adult goldfish. Goldfish commonly reach sizes of 8 to 10 inches, making them far too large for a 10-gallon tank. Fancy goldfish require at least a 20-gallon tank, and common goldfish need a 30-gallon tank.
The minimum tank sizes are recommended for a single goldfish, and the owner should add 10 to 12 gallons for each additional fish. For instance, a tank housing five fancy goldfish should be at least 60 gallons in capacity. While goldfish are hardy enough to survive in smaller conditions, they do not thrive or reach their full adult size in small tanks.