Thursday evening I tested my pool for chlorine and pH. Normally I see a variation of a few tenths of a ppm change from day to day. However on Thursday I had a drop of almost 1 ppm! This worried me and go me to thinking about why the drop. I then realized that the day before I filled to pool. I fill with tap water, and in my area, tap water usually runs around 3 to 4 ppm of CC.
So it looks like the CC in the fill water dropped my FC by 1 ppm. I am going to have to remember in the future to add 1 ppm FC when filling the pool.