Pond fish, like all living creatures, require that certain conditions in their environment be met to sustain their health in ponds. Koi, due to their mature size and their longevity, need a little more care than goldfish.
Sometimes health problems are introduced by new pond fish or develop because conditions in the pond are less than ideal. Excellent fish medications developed specifically for Koi and other pond fish are available for most pond fish ailments, but care must be taken that dosage requirements are observed.
Pond medications, unlike human treatments actually treat the pond water and not the koi, gold fish, etc. The dosage applied to the pond water is determined by the volume of the pond rather than the size or number of fish.