Been treating black algae in our plaster pool. Brought the FC level up to 35-40 ppm (all other levels OK after correcting CYA levels) and kept it at that level for 6 days with less than 2 ppm loss overnight every night.
Started letting the FC level drop on it's own 3 days ago. CYA is 50.
Day 1: FC overnight dropped 2 ppm to 36 ppm, during the day dropped another 2 ppm to 34 ppm. (We don't get a lot of sun this time of year plus the weather was cool - upper 70's.)
Day 2: I dropped the pump from 24 hours a day to 16 hours a day split between morning and evening. Daytime temps up to low 90's. No drop in FC overnight but 5 ppm drop during the day down to 29 ppm.
Day 3 (yesterday): Dropped the pump to 14 hours. Daytime loss of 7 ppm down to 22 ppm. Weather hot again.
Day 4: (today) This morning when I tested the FC I got a 3.5 ppm drop to 18.5 ppm. WHAT I said??? The pump had been off for 5 1/2 hours when I tested the water.
So I turned on the pump and then tested the water from 4 other locations in the pool.
Got readings ranging from 19 ppm to 20.5 ppm. So I am looking at a high end of 3.5 ppm overnight loss (18.5 ppm )to 1.5 ppm overnight loss (20.5).
Which reading should I use as my guide? Is it typical for the levels to vary that much from one part of the pool to another?
15,000 gal white plaster IG jet vac sweep new pumps and cart filter
FC 18.5 to 20.5?
pH ?? too high to test