Very slow chlorine drop after SLAM


New member
May 28, 2020
Chesapeake VA
I SLAM'd my pool on September 9th to clear up mustard algae and it worked beautifully. The water cleared up within 24 hours and has stayed clear since. I added 8 lbs of calcium hypochlorite per Pool Math app, which brought FC up to 35 and I turned off the SWG cell but ran the pump/filter continuously. Two days later FC had dropped to 25 but then really started to drop slowly. One week later FC was still 14 and on 9/22 it was 8. CYA has been steady at 60, TA also at 60 and pH is at 7.5. Is it unusual for it to take so long for FC to drop? I'm shooting for FC of 4.5 based on my CYA of 60. My pool is pretty shaded by trees and nobody has been in the pool since the SLAM so perhaps that explains why the FC drop is slow. I'm wondering if I should do anything different if I need to SLAM the pool in the future.


TFP Expert
LifeTime Supporter
Jan 6, 2010
San Dimas, CA (LA County)
As the weather (and the water) cool and the days get shorter, you'll find that chlorine lasts a lot longer. Here on the west coast where we don't close, come January I'll add in a week what I used to add daily.

Enjoy it. The sustained higher chlorine is ensuring a good kill on that mustard algae.
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