I have a 9'x9' liner pond with a 10"- 12" deep shelf, with the pump/fountain in the deeper center (about 20 " deep), connected to a submersed box filter. I (mostly) understand about moving the pump to shallower area during winter so as not to disturb the warmer water on the bottom for the goldfish and rosy minnows. I am wondering about the box filter...what is its purpose during the winter what maintenance does it require? Thanks a lot