OK, for the past couple of years -- in the midst of this California drought -- we've been making do with a pool that has an extremely high CYA because we didn't want to drain our pool. I forget the exact numbers (tested everything at the beginning of the season) but we were supposed to keep the chlorine levels high, at about 14-17, and we did so with no problems. No skin irritation, and all our other numbers except the sky-high CYA tested in a good range.
According to my husband, he used on average about 6 cups of 8.25% bleach a day.
Then we left town for the holiday weekend, and our pool was left "refilling" (read: overflowing)...for three days. When we came back, and saw what had happened, I tested all the numbers, and it was as if we had changed almost all the pool water. So, I got all our numbers back in range (added some borax and calcium) and saw in pool math that with our new CYA levels of 40, we want the chlorine to be between 3 and 7.
When we tested it after coming home from our holiday weekend, it was down to about 2 but the water was constantly moving (refilling / overflowing). No sign of algae. My husband added about a half gallon of 8.25% bleach. the next day it was still 2. He added a gallon of 8.25% bleach. That morning it went up to 6, but by the afternoon, it was down to 2 again.
To go from 6 cups a day to a gallon a day (and still not maintain the chlorine level) is very strange. We get pretty much sunlight, but we also have a good bit of shade before 9 am and after 3 pm. Would the low-ish CYA account for quick disappearance of the chlorine?
We'd figured out the pool perfectly with the high CYA and now it feels we're starting form scratch.