I have a SWG unit but have been running my pool without it using bleach and had no problem.
I wanted to start using the SWG so I have added salt to a level of 3000 ppm.
I have also added CYA to raise its level from 30 ppm to 40 ppm, but this one is not dissolved yet.
After few hours I have tested FC and was suprised to see that it dropped to 0, CC was 1.5 ppm.
I have added bleach to raise FC to 5 ppm but after few hours FC dropped back to 0.5 ppm.
I have added again bleach to raise it to 8 ppm, tested after 15 min and it was OK. 2 hours later FC dropped again to 1.5 ppm.
During all this time I had the SWG running and the additional CYA soaked in the skimmer.
pH is 7.4, TA is 400, CH is 360.
It is an outdoor pool but all this happened after 5pm so no direct sun light.
Any idea why FC levels are dropping?