You've constructed a beautiful pond and installed water plants. What's missing? The languid movements of gleaming, golden fish that could become tame enough to feed from your hand. More and more Canadian backyards are playing host to Asian ambassadors. These exotic emissaries are beautiful and graceful, yet surprisingly hardy. Some reportedly live more than a century. Called Nishikigoi or "living jewels" in Japan, we know them as koi. Popular as ornamental pond fish, koi come by their toughness naturally. They are descended from Cyprinus carpio, the common carp of Europe and Asia. Wild carp cope well with trying conditions, including pollution and oxygen-poor water, surviving where more sensitive species would perish. Carp also tolerate variations in water temperature better than many other fish. Appreciating their hardiness, Japanese rice farmers reared carp as food fish. During the 1820s and '30s, they began to select the more colourful fish and breed them for increased beauty. Interest in these living pieces of art spread from the farmers throughout Japan. After Crown Prince Hirohito received a gift of koi for the Imperial Palace moat in 1914, demand for the fish grew dramatically. Dramatic growth is also one of the reasons people love keeping koi. A koi may reach 20 centimetres (eight inches) in length in its first year, and up to twice that length in its second. Koi may grow to 90 centimetres (three feet) long and weigh 20 kilograms (44 pounds). However, mainly due to the small ponds in which they are kept, the average length of a mature koi is closer to 50 centimetres (20 inches). Another popular brightly coloured pond fish appeared on the scene well before koi, but it is also descended from a wild carp species. The ubiquitous goldfish-whether fantail, comet, red cap, globe eye, or any of nearly 300 varieties-was deliberately created from the gibel carp (Carassius auratus). In its native China, the gibel carp survives in varied water conditions, from running rivers to stagnant ditches. The first record of the golden form of the gibel carp comes from the Chun dynasty (AD 265-419). By about AD 900, the golden form had been established and the Chinese were selecting and breeding for other colours, fin shapes and sizes, and scale types. Goldfish reached Japan in the early 1600s and the Japanese furthered the selective breeding process. Collectors and hobbyists slowly spread the results of their efforts around the world. Remember the "burials at sea" when your pet goldfish died? Apparently, some of them still had a pulse. Goldfish have made themselves at home in the continent's lakes and rivers, the first non-native species to do so. They have been collected from every U.S. state except Alaska. A recent poster showing the fish of Manitoba contains an unexpected but familiar finny face. Which fish will work best in your pond? For most Canadians, goldfish are the best choice. They grow less than half the size of koi, topping out at 25 centimetres (10 inches) in length, and are happy in a small pond. A smaller fish also requires less space indoors over the winter. Goldfish need only a simple pump or airstone to circulate and aerate the water when temperatures climb above 25 degrees Celsius and the water's oxygen level drops. Koi do best with a specialized continuous filtration system. Goldfish will nibble some pond plants, but koi are gluttonous grazers that may wreak havoc on pond greenery. In addition, you can purchase goldfish from many sources, including pet stores and water gardening specialty shops, priced from 99 cents to $50. Koi might cost 100 times that, depending on the variety and size. Both goldfish and koi will enhance the placid beauty of your pond, and work hard to keep it free from another foreign species: West Nile virus. Both fish are avid consumers of mosquito larvae. No wonder the ancients kept goldfish in ponds around the house to bring good luck. - REBECCA L. GRAMBO