Winter is the biggest threat to a pond; in the winter there is ice. Ice expands and can burst pipes and filter housings. Be sure to remove all points of the system where pressure can accumulate. Remove filter pads and lids from filters, allow for pressure relief in the system where possible.
The freezing of the pond surface can create dangerous conditions for fish due to the accumulation of gas below the ice. An ice preventer or an aerator should be used in these cases to allow for the escape of toxic gas and proper oxygenation of the water.