Rapid FC drop after adding Calcium Chloride


May 26, 2020
Austin, Tx
Pool Size
Hi everyone,

I’m looking for some help/advice.

My daily chlorine levels are typically pretty consistent and require that I add somewhere between 20-30oz of liquid chlorine (1-1.5ppm). I did my weekly chemistry tests yesterday and everything looked OK. My CH levels have been running a bit on the lower side, so I decided to go ahead and add Calcium Chloride to get the numbers up to a better spot. In total I added 14lbs throughout the day.

Before adding the Calcium Chloride my FC was sitting at 3.5. This morning I went to check the chlorine levels and I was at .5. I was surprised by the big drop overnight, but to get the levels back up I went ahead and added 80oz of chlorine to get me back to the 4.0 range. I checked the water again about an hour later and it’s still sitting at .5ppm.

I tested with two different sets of reagents and even took a water sample to Leslie’s to have tested. All of the results were consistent. The water is clear and I don’t see any signs of algae.

The only thing that I did out of the ordinary yesterday was adding the calcium. Is there any reason that would drop my FC level? Any recommendations on next steps to get the chlorine levels back up?


Mod Squad
TFP Expert
Bronze Supporter
May 3, 2014
Laughlin, NV
Pool Size
Salt Water Generator
SWG Type
Pentair Intellichlor IC-40
You reported the same effect in January. Odd. Adding calcium chloride will not effect your chlorine level.

We can assume you now have the TF100 kit? Signature says it is on order.

EDIT - your FC levels in Poolmath are consistently below minimum. That is not a good way to operate. Do an OCLT.
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