A pond is an ecosystem that is constantly influenced by natural forces such as sunshine, rain and temperature, but also by the change in seasons, debris, leaves and dust that land on the water surface. Filtration can help maintain the pond ecosystem's balance in order to create an environment in which your plants and fish can thrive.
A few things should be considered when choosing a filtration system:
The effective pond size for a filter is commonly based on a pond without fish or Koi. Once fish are added to a pond, the required filtration capacity increases, meaning a larger filter system is warranted. With Koi, especially when the number of Koi to pond volume ratio is high, the required filtration capacity can easily be 4x the actual pond size.