Green Algae Keeps re-appearing


Well-known member
May 23, 2007
I keep shocking with liquid chlorine but the green Algae keeps coming back. Right now the pool is clear but when I get home from work there is Algae on everything lightly and some thick bunches in spots. Any recomendations on what to do?

20*40 Liner pool with 8.5 foot diving end (Think it's about 25K)
Saltwater with filter now on 24Hrs with generator set at 60%

Before adding anything tonight these were my readings:
FC 5
CYA 50
PH 7.8
TA 210
CH 290
Water temp is 82

I added 1.5 gallons of 10% chlorine and .5 gallons of Muratic acid


TFP Expert
LifeTime Supporter
May 30, 2007
South Carolina
It does not sound like you are reaching shock level based on your CYA level.

What test kit are you using?

Do you use Jason's Pool Calculator for dosing? (link in my sig)

Shock level for your CYA of 50, is between 16 & 20. Adding 1.5 gallons (192 ozs) of 10% bleach will raise your FC by 6ppm, assuming 25K pool. If your FC was 6 ppm when you tested and added the 1.5 gallons, you would now have only 11ppm FC which is still NOT shock level. :(

I suggest you see the Pool School articles on "Defeating Algae" and "How to Shock Your Pool".
Let us know if you have any q's. Lots of folks around to help. :)


Mod Squad
TFP Expert
Platinum Supporter
LifeTime Supporter
In The Industry
Apr 1, 2007
Sebring, Florida
I suggest you see the Pool School articles on "Defeating Algae" and "How to Shock Your Pool".
Double ditto!!. Shcok according to the process described in Pool School and your algae will be gone.

FWIW, if your pool is 20'x40' with a diving end, you have a lot more than 25,000 gallons....closer to 35-38k

chem geek

TFP Expert
LifeTime Supporter
Mar 28, 2007
San Rafael, CA USA
If the algae tends to come back in the same place, then perhaps circulation is poor in that area. Try pointing returns in a somewhat different direction. You can also measure the FC near the area of algae growth as that is probably the lowest FC point in the pool. Also, be sure to brush the pool, especially where algae is present, and not just shock -- one has to get rid of any biofilm that has formed and brushing helps with that.

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