Basically the only way to guarantee that it won't degrade is to make sure your pool stays algae free all winter. I know--easier said than done!! Some things that might help would be shocking the pool to make sure there is no existing algae, then using a good dose of Polyquat algaecide, followed by shocking the pool again just before closing, to try to eliminate as much algae as you can. If you are able to uncover the pool to pour bleach in during the winter and circulate it, even if you have to use a broom, paddle, etc. , that will also help. Also, if you'll wait until you absolutely have to close the pool in order to close it with the water as cold as you can, and open early, while the water is still cold, that should help some, too.
I don't close my pool, so I'm sure that others that do will chime in soon with other suggestions, but maybe these will get you started...
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