Koi are kept in ornamental ponds worldwide but often suffer from poor water quality, inadequate space, and lack of veterinary care.
Koi in undersized ponds with inadequate filtration experience chronic toxic exposure from ammonia and nitrite. Overcrowding causes immune suppression and disease. Temperature extremes in shallow ponds stress fish that have evolved for deeper water. Predation by herons causes repeated acute fear responses even without successful kills. As intelligent fish with long lives and individual personalities, their welfare deserves serious consideration from those who keep them.