About 10 days ago, after some serious rains, I noticed a rapid drop in FC along with some very pale yellow growth around the skimmer (slimy too). I proceeded to follow the shock process (first time I have needed to do this). Got up to shock level and continued for several days until I saw no visible overnight loss in FC and the pool water looked really good. Later that day, I did another reading and the FC level had dropped from 12 to 7. I figured that I had not been successful, so I added enough bleach to get me back to 12 that evening. This morning's read was 12. All good, water looks great. This afternoon, the reading has again dropped to 7. I am puzzled! Any thoughts?
Pool readings as of now:
FC - 7
TA - 80
CYA - 30
CH - 230
PH - 7.2 (this reading taken before I started the shock process.
Thanks in advance for your thoughts.