Once the water gets really cold, there's no chance for algae growth, or need for more chlorine. Algae has alot of trouble growing in water below 60deg, and is more and more inhibited as it gets colder. Rule of thumb is that algae doesn't grow AT ALL below 50 deg. Since I'll get a foot of ice or more on my pool, I know the water is in the 30's--no algae growth there!

The idea is to keep algae from starting between the time you close and the time it's too cold to grow. The heavy dose of PolyQuat further inhibits algae growth and the shock level kills anything there. The covered pool tends to hold chlorine levels far longer because there's no ultra violet getting to the water. Even the mesh covers work very well at this (I have a mesh safety cover).

Once I close, all I do is check the water level once a month and syphon off water if it's too high.