Pool turning green despite FC at recommended level

GearHead36

Well-known member
Jun 18, 2015
77
Madison, AL
Our pool is in N AL, and has been open all Winter. We've been running the pump for 4 hr/day for most of the Winter. Recently, it's gotten a bit cloudy, so I upped it to 6 hr/day. The weather is warming, and the pool is turning green, despite the FC level, which we've been maintaining.

The numbers (using a TF test kit) are:
FC: 3.0 (rec:5-7)
CC: 0.5 (rec:NA)
TA: 40 (rec:50-90)
pH: 7.4 (rec:7.2-8.0)
CYA: 40 (rec:30-60)
temp: 66.7°F

Yesterday the FC was 5.0. The weather and algae are obviously using up chlorine, but I don't understand why the algae started growing with a maintained FC level.

Over the Summer, pH tends to rise, so I typically start it out on the low side. Once it gets to around 7.8, I'll adjust it.

Thanks.
 

mknauss

Mod Squad
TFP Expert
Bronze Supporter
May 3, 2014
37,027
Laughlin, NV
Pool Size
6000
Surface
Fiberglass
Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-40
Sounds like you need to follow the SLAM Process

Have you been brushing the pool regularly all winter? Brushing is required to help keep algae from forming.
 

PoolGate

Gold Supporter
TFP Guide
Jun 7, 2017
5,544
Damascus, MD
Pool Size
29000
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Jandy Aquapure 1400
Are you targeting the minimum chlorine level for your CYA reading? I target and suggest you target 10% of CYA.
 

mguzzy

Gold Supporter
I bet you had some residual algae in the pool over the winter since you didn't close it.. and now that the conditions are ripe for growth (Higher sun, warmer temps) they are growing faster than your FC levels are killing it. I bet an OCLT will confirm that. Its not uncommon to have to start the season off with a SLAM when the pool only really had a soft close over the winter. I fell into that trap all the time before my TFP days.
 

ajw22

Gold Supporter
TFP Guide
Jul 21, 2013
24,658
Northern NJ
Pool Size
35000
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-60
The numbers (using a TF test kit) are:
FC: 3.0 (rec:5-7)
...
CYA: 40 (rec:30-60)

FC of 3 is the minimum FC level for CYA 40. If you are running your FC at the minimums it is more likely your FC fell below the minimum levels at some points in time and algae took hold. Once algae takes hold normal FC levels may hold it in bay but not eradicate it. That requires SLAM levels of FC.

It is better to keep your FC at the high end of the range and run it hot rather then skirt the minimum levels. Nothing bad happens a few PPM above recommended levels. Fall below the minimum FC level by 1 or 2 ppm and you can end up with a green pool.

It is time for the SLAM Process. Then run higher FC levels.