Want to close but new problem

Hi,

I live in western PA so I need to close my AG about 17,000 gal pool. It's already getting pretty chilly at night now.

I turned off the heater about 10 days ago, covered it with the solar cover and a net leaf cover and had to go out of town. (Water temp went from 82 to 60. Brrrr.)

I've had balanced water all season with never any CC. Now my latest reading shows that I have a
FC - 4
CC - 2

Why would I have that now? No one has used the pool since I turned the heater off. Is that from an impending algae growth or the result of the temp falling? My FC/TC has hovered around 4 the whole time. (I've added bleach a couple of times to keep it there.)

I know I need to shock it prior to closing anyway. The catch is, I ran out of CYA testing liquid so I can't test for CYA now so I don't know how to calculate accurately using Mike's table (thank you for sharing that Mike). Last time I ordered some it took over a month to get, I apparently didn't order enough, and no one around here carries anything but the strips. Per them my CYA is in the 30-50 range.

This past spring I opened to green sludge even with the use of Polyquat last fall. I'd like to avoid that next spring if possible.

In what order to I add things to get rid of the CC? Shock first then add Polyquat? Do at same time? Polyquat first? If I shock and then put pump on recirculate for the polyquat, does it negate one or the other?

I will be putting a solid cover on it this year unlike the past where I used a mesh tarp. Watermom mentioned leaving the cover off for awhile after shocking to vent the fumes but how long does that take and won't the TC fall to below shock value while that's happening?

As you can surmise, chemistry wasn't my strongest subject in schools. Am I missing something I should be doing?

Thanks for the help.