FC drop overnight


Well-known member
Dec 25, 2019
Slammed my pool about 10 days ago last week as noticed that it was going cloudy (not green) but more hazy. Also noticed first thing in the morning my FC was at 0 at that time after it was 5 or 6 the night before.

Slam started and passed the OCLT. Water was crystal clear.

Fast forward to today and Pool has drifted down to 12 yesterday after the slam and had a pool party. When I removed cover I noticed it was less crystal clear. Started my SWG yesterday too as FC had drifted down.

Last night once cover was back on, FC was 10.5. Tested this morning and it was 8.5.

Is it normal for the FC to drop overnight like this, especially after a SLAM. This is my first swim season as a first time pool owner.

My results are below.




Build Type: Plaster

Volume: 60000 liters


Latest Test Result Summary:

FC: 10.5 (14 minutes ago)

CC: 0.0 (5 days ago)

pH: 7.4 (5 days ago)

TA: 90 (5 days ago)

CH: 360 (5 days ago)

CYA: 60 (6 days ago)

SALT: 3600 (11 days ago)

BORATES: 4 (22 days ago)

TEMPERATURE: 26° (6 days ago)

CSI: -0.32 (14 minutes ago)



Gold Supporter
TFP Guide
Jul 21, 2013
Northern NJ
Pool Size
Salt Water Generator
SWG Type
Pentair Intellichlor IC-60
What type of pool cover do you have?

My guess is your pool cover has algae on it and is introducing algae back into your water after the SLAM Process is over. Your cover needs a good chlorine bath and soak.
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