I just increased my calcium hardness from 150 to 200 and noticed a marked increase in chlorine usage. Chlorine actually went to zero by days end. Which has only occurred once before, early this year, when I also added calcium (pool store's Hardness Plus). Thought the first time was just a coincidence, maybe I had the beginnings of an algae bloom. But, now that I've seen it again, I'm beginning to wonder. The first time, I just raised the FC back to 5 and all was good (never shocked it). Trying that again now.
Chlorine has been above the minimum (keep it around 5, my CYA is ~40), so doubted I was fighting algae (but hard to know for sure). Full results:
FC: 5.5 before adding CH in the morning, 0.0 days end
CH: 150 before, 200 after
I did a quick search, and found some similar topics:
So, guess I'm wondering how many others have seen this. From the search, doesn't seem to be a common problem, but figure I'd throw it out there.