I have a SWCG and normally try to run my CYA level between 60 to 80 ppm. During the last couple of months, I noticed that to maintain an FC level above 3 ppm I had to increase the setting on the SWCG. At the time I thought that this was due to the summer sun.
Like just about everyone, I have had trouble getting accurate readings on the CYA test. The readings depend on many factors including ambient lightin conditions. I check the CYA level at the beginning of each month. Last month, for some reason the reading was very low ~30 to 40 ppm. I promptly added enough CYA to bring the level up to 70.
Anyway my FC level jumped from 3 to 5 ppm in less than one week after adding the CYA. This is dramatic proof of the effect CYA has!