I have researched AND researched pool closing chemistry. After due diligence, here is my recommendation for IG vinyl liner with solid cover, freezing winter, and algae-free balanced water:

1) 2-3* nights before closing: Vacuum/brush and backwash if needed. Bring to shock level.

2) 1-2* nights before closing (next night): Add 60% polyquat at "visible algae" dose.

3) Night before closing: Check floor for debris, check filter pressure; adjust as needed. Raise FC back to shock level. Circulate 2 hours and lower level to below skimmer.

4) Close mechanics however you wish.

* Some people differ as to waiting 24 or 48 hours after polyquat addition. If waiting 48 hours, check FC after 24 hours to be on the safe side. Polyquat will lower fC.