Koi carp are popular ornamental fish but their welfare in garden ponds is often overlooked, with issues including poor water quality, overcrowding, and inadequate nutrition.
Many koi owners underestimate the space and water quality requirements of these fish. A common rule of thumb is 1000 litres per fish, but large koi need considerably more. Inadequate filtration leads to ammonia spikes that damage gills and cause chronic stress. Koi demonstrate clear preferences and aversions, suggesting a meaningful capacity for positive and negative experiences that owners should take seriously.