Your circulation pump should normally be run 24 hrs per day, 7 days per week, 52 weeks per year unless you have hard winter freezes. This would always be true if your bio filter is in line with this water flow. The good bacteria begin dying within hours of loss of oxygenated water flow. Also, oxygen levels can drop dangerously low in a pond, especially at the bottom overnight. So, unless you have a separate aerator which is running, you should leave your water circulating continuously to prevent this as well.