You need to raise pH. To figure how much of what to add, you either need to test TA or use the Base Demand test.
It probably wouldn't hurt to run ALL the tests, and enable the results to be shared so we can see something on your logs besides
"pool at the lake." ( I kinda doubt it's 210,000 gallons , too) With pH crashed like that I wonder if you've been feeding it a lot of acidic trichlor pucks and dichlor shock.