My husband is happy to have a game plan("I told you we should drain the pool!")

Yes, we put the cartridge back in.

More pool details:

concrete reinforced fiber glass wall with a steel mesh reinforced floor installed in 2003

Filter: Sta-Rite Series III 450
Natural Gas Heater: replaced this year Sta Rite 400 btu/hr
AO Smith Replacement Motor 1.5 HP (replaced 2008)
PB 4 Booster Pump
Hayward Chlorine Feeder CL 220(Replaced 2006)

Nature 2 Cartridges
Polaris
Pool was painted one coat 2007

He is shocking the remaining water now. Will keep you informed. thanks