Fish should be treated with a general medication at least twice a year, essentially at the beginning and end of the summer. This is to stop the development of internal bacteria and also reduce the likely hood of external parasites that become present in all ponds at one time or another, normally brought in by birds or other wildlife. It is also recommenced to do small 10 - 25% water changes during the early years of a pond to help with biological establishment of the filters and the overall chemical condition of the pond. The solution to pollution is dilution.
With a pond the key to trouble free fishkeeping is to manage and maintain water quality, Regular water testing and maintanence will ensure many hours of enjoyment from your pond.