One of the most recognized fish in the world, goldfish have been bred for centuries; this one species comes in many colors and variations. Koi are grouped with goldfish but actually related to carp. Fancier fantail varieties of goldfish generally do best in tanks, while streamlined comets and koi are great for outdoor ponds.
Goldfish give off a lot of waste. To prevent water from becoming polluted too quickly, allow no more than one inch of goldfish for every two to three gallons. Species that stay small, like the China Doll variety, can be kept in a bowl but need at least two gallons of water. These fish act hungry at all times, but should never be overfed to avoid producing even more waste.