I leave the pumps in my two ponds running all year round. In late Autumn the pumps are raised onto up turned planting baskets and put back down to the bottom in Spring, as soon as the water temperature rises. The water has always got a clear bit under the water falls, and they look spectacular when ice forms in them and on the adjacent plants. Both ponds have a fairly high stocking rate of healthy, happy fish which produce young, and they are also home to a multitude of frogs that patrol the garden during the summer keeping the slugs under control. The advantages of ponds in gardens are many, the diversity of insects that are attracted to them is wonderful, if you haven't got one , get a spade and start digging.