When closing down your outdoor swimming pool for winter you need to create a sterile environment where micro-organisms such as algae spores, fungi and bacteria cannot flourish. You also need to help prevent the formation of scale and staining from dissolved minerals so the water and pool surfaces are in good order when the pool is re-opened inspring.

If you can do this, you should hit the ground running at the start of next season as far as water quality is concerned, and avoid those expensive and time consuming clean-downs which are needed before the water begins to look acceptable again.