low FC again - rain or something else?

[keep-it-simple]

All this testing and balancing makes me

Questions:
Dropping chlorine - effect from heavy rains or indicative of need to shock?
High TA - acid and aerate?

Test Results:
June 12 after 2+ inches of rain
FC - 1
pH - 7.1
TA - 70
CYA - 70
Added bleach. FC up to 5 (tested 5 hours after bleach from arm's length depth at the side of the pool opposite where bleach was added)
Dumb me forgot washing soda adds TA and I brought the pH up to 7.5 (and TA up over 100)

June 13-16
out of town, no tests, no swimmers other than the plethora of beetles that fall from the pine trees

June 17
heavy swim load of young kids, 1.5" of rain, no tests, SWG running at 50% for 12 hours

June 18
Backwash 2" from pool (roughly 800 gallons)
FC - less than 1
Added bleach. FC up to 5.
It continues to rain another .5" or more of rain throughout the 18th after adding bleach.

June 19
FC is down to 1 again.
pH - 7.5
TA - 100
CYA - I'm out of reagent

I have CYA reagent on order.
I have acid available if needed.
Should we shock or wait and see if Chlorine level holds after the rain lets up?

 

[Bama Rambler]

As a normal rule, rain makes very little difference in the pool balance. It may raise the pH a little but that's about all. I think you have something else going on.

I would raise it to shock level and turn off the swg and run an OCLT.

 

[Smykowski]

You let your FC get too low, too often. Per the pool school chart, you should never let FC go below 3.

Shock it.

 

[keep-it-simple]

Thanks, both.

FC maintained at 5 previous to now. All tests were within appropriate range since the pool was balanced back in March after opening routine.
June 12 was the first low test so whatever is going on is new, and seems to be causing the low FC rather than due to "allowing" low FC. The water is clear and beautiful so hopefully oclt is 0 after shocking.

 

[sarahspins]

We had a lot of rain a few days ago and ended up with a lot more dead bugs on our pool, and my FC did take a bit of a hit as a result (dropped to 4, I normally keep it at 6-7), but it didn't drop below what is safe for my pool, it was just lower than it had been. I added some bleach to make up the difference and it's been fine since.

When you let your FC drop to basically 0 (however it happened) and you had swimmers before testing or adding more chlorine, you are probably now in the early stages of an algae bloom... it's going to use up what FC is in your pool pretty quickly, and not hold a stable level like it had been. It's kind of a chicken vs egg problem, but however it happened doesn't matter, the solution is still the same.

If it were my pool, I'd shock just because the chlorine went low the first time - I wouldn't mess around and wait for it to get worse.