Quick update -

I vacuumed this evening (all the big stuff lying on the floor) using a skimmer sock, then removed the cartridges and replaced with clean ones. Afterwards, the water was tested and I got:

TC - 3
Ph - 7.6/7.7
TA - 110
CYA - assumed to still be at 90 (tested yesterday)

Using a test strip for free chlorine, I got 3.

After a little hesitation (since we are using a Nature2 cartridge), I put in 10.5 gallons of bleach (6%) to get to 23ppm, which is the approx. shock level for CYA at 90 according to table posted in this forum.

Am now running the filter 24/7.

Hope to have more news (good or bad) in the morning. (I think I'm using parentheses too much?)