To be changed
If you keep fish, the pump and filter must be run constantly, all year long. Water movement from the pump (via the fountain or waterfall) increases essential oxygen in the water for the fish. The friendly bacteria within the life support system that processes fish waste and maintain a healthy pool (see How to introduce fish successfully) also require a flow of oxygen rich water to live. If the ‘life support system’ is turned off for more than a couple of hours, oxygen levels will drop and the population of friendly bacteria will be reduced. This can lead to water health issues and poor conditions for your fish.
Providing your pond does not freeze to the bottom, therefore freezing the pump, you should run your pump and filter all year around.