Personality: Koi are gentle, peaceful and mellow. Very large Koi can share a pond with much smaller Koi with no danger.
Care: Koi are hardy fish that can live 30-50 years, according to Koi and Ponds. (The record stands at 233 years!) The most important aspect of care is quality of water. If water quality is poor, the fish can die quickly. The water must be in a pond (300 gallons per mature fish, and at least three feet deep) that remains a proper temperature and has filtration to keep the water clean. Nutrition is also extremely important, Koi and Ponds says. Koi will eat just about anything, but a proper diet will help them stay healthy and grow more quickly.
Common Health Issues: Though generally healthy and long-lived, koi are prone to a number of diseases. Like goldfish, they can develop fin rot and Ich. Other health issues to watch for are anchor worm (parasitic bacterial infection), cotton wool disease (bacterial infection), dropsy (sign of another condition or bacterial infection), fish lice (parasite), gill maggots (parasite), Saprolegnia fungus (fungal infection) and skin and gill flukes (fish equivalent of fleas), according to The Koi Farm.
Training Tips: Many koi owners have success in training their koi to eat from their hands, according to eHow.com. Keep in mind, however, that once your koi learn to accept food from you without fear, they might come to the surface for other animals, including predators like herons.