Garden pond fish including koi and goldfish have complex welfare needs related to water quality, space, and compatible stocking that outdoor pond keeping must address.
Ornamental pond fish welfare is determined by water quality above all other factors: ammonia and nitrite from inadequate filtration cause chronic immune suppression and fin damage that accumulates over years. The scale of the pond and quality of filtration system must match the fish load — under-filtered ponds with many large koi are chronically welfare-compromising. Secondary factors including predation protection, appropriate water depth for temperature buffering, and compatible species stocking all influence individual fish welfare continuously throughout their long lives.