Left covered all day with minimal sun. No pump run through night and day. First time in 3 months
4:30
FC 10
FC .5
You don't have any sort of cartridge or ozonator or extra filter or other gizmo installed, do you?


I'll let you know what I decide about the tests you want to run.
Uh, you might have not meant that exactly, but we don't have any particular desire to run them. It's just that -- unless there's something you've overlooked or forgotten to tell us -- there's nothing else we can do to explain your situation.

As far as we know, chlorine in an unused swimming pool with CYA=100 does not drop like that, unless bromine / bromide or iodine is in the pool.