I have seen this as well. My pH started at 7.2 with the fill water. It came up to 7.4 after a night of rain, and hasn't moved since. I'm using the Intex 1600 gpm pump with 14" sand filter/SWCG combo. I over-shot the salt and am at 4400 ppm, TA = 120, CYA ~ 60. I run the SWCG for 2 hours/day and FC holds right at 8 ppm consistantly.