When washing the filters, always wash them using water from the pond to avoid killing off beneficial bacteria. It helps to think of a pond filter as a sewage works from a pond – the most important job is the bit you can’t see, using natural bacteria to break down harmful substances.
Through winter, you can slow down flow by 50% using an inline valve or pump flow controller and raising the pump away from the base. This will avoid overchilling the water in the pond because water is warmest at the base. Also, there are new models that have e lectronic flow controls to decrease the running costs over the colder months when you don’t need the full flow.
Ensure you never smash the ice on a pond because the shockwaves can harm the fish. Instead, what I do is use a kettle of boiled water in one area to melt the ice gently to create a hole in the ice so oxygen can get in and harmful gasses can escape.
There are other longer-term solutions such as products that were designed for this purpose that will work throughout winter to prevent freezing. For example, Oase IceFree 4 Seasons keeps an area of the pond free of ice in temperatures down to -4˚F.