Treating your pond with the proper additives will greatly increase your chances of a clean, clear and healthy pond in the spring. Nitrifying bacteria (good bacteria) is what helps keep your pond clean. The problem, is, when the temperature drops below 55 degrees, it usually dies or goes dormant. Use a specialized product like Microbelift Autumn Prep which will continue to provide sustained biological activity in water temperatures down to 38 degrees water temperature. It also helps to jump start your pond to a healthier environment in the spring. Add a stress coat like Pondcare's Stress Coat to help protect fish in the colder weather.
In the fall it is essential that you remove all the debris and scum from the bottom of the pond, as this will make the difference in water quality and clarity in spring. Use a Pond Vacuum or a Skimming Net to get the job done.