Richard's right. I close by waiting till the water is 60 deg or less (if practical), shocking my pool up to the CYA shock level, adding 1 quart of Polyquat, waiting 48 hours (Polyquat drops FC levels), bringing it back up to shock level, and closing. I use a safety mesh cover. If we have a good, cold winter I'll get a foot of ice on the pool. By mid-April, when the ice melts, if I open then, I usually do NOT have algae. I've only had algae the last two openings--because I opened late last year and this winter was warm. But it was cleaned up very quickly because I don't hesitate to hammer it with LC, usually in 24 hours.