Pool seems crisp as long as the HOCI is above .05 ppm


Well-known member
Jun 22, 2007
Atlanta, GA
It seems that when I shock my pool, it looks fresh and crisp. As the FC drops to normal levels, it crosses some threshold and loses it's crisp look. I'm guessing that line is .05 HOCI, because after my pool dropped below 10ppm FC, that's when I noticed it wasn't as crisp. The spreadsheet says I need 9ppm FC to get .05 HOCI. I'm going to push my chlorine back up to 9ppm and see what happens. What do you think? Could this be an algae problem still? I just brushed my pool, like I do every weekend. This time, there was some brown dust that would brush up in many places.

FC 6
PH 7.4
CA 370
CYA 80
Temp 85
Borates 50
Phosephates 1000+

chem geek

TFP Expert
LifeTime Supporter
Mar 28, 2007
San Rafael, CA USA
Every pool is a little different. In my pool, the "crisp" threshold appears to be around 0.02 ppm HOCl (or maybe 0.03) but since you've got lots of phosphates in your water, maybe 0.05 is the threshold for your pool. The 0.05 is roughly the mid-point in Ben's original table so probably works for most pools. We know that yellow/mustard algae needs higher chlorine to keep away -- about the 0.07 column. So perhaps in your pool you'll need to keep the chlorine at the 0.05 level.

As for whether you still have algae trying to bloom, you can see the overnight drop in the FC level. Chlorine consumption and "crisp" vs. dull water can be not only from algae, but from particulate matter. Not only will higher chlorine break some of that down faster, but longer filtration or better filtration (i.e. some DE added to a sand filter) might help. Anyway, those are some things to try.



LifeTime Supporter
Apr 5, 2007
Joliet, Il.
I agree with the others suggestion to check your overnight FC levels. Instead of shocking your pool water up to a certain level and then letting it drop, shock up to that level and sustain that level until your free chlorine levels hold overnight.