Holding FC levels overnight and possilbe organic stain


New member
Jul 14, 2009
Southeastern Pennsylvania
When we opened our pool this spring there was a large stain in the deep end around the main drain. We regularly get a large volume of worms that enter the pool after we close it in the Fall that must be vacuumed out when we first open in the spring. We assumed the stain came from the worms and/or possibly leaves that sat in the deep end over the winter. We have rubbed Vitamin C tablets directly on the stain and none of it came off. The stain appears to have lightened since opening the pool but is still very visible. Brushing the stain does nothing to it.

We are now trying to verify if the stain is something living that can be removed by shocking the pool. We use standard store bought bleach to sanitize the pool and have attempted to keep the level above 3 ppm since opening. We believe we may have never shocked or sanitized the pool enough since opening this year to kill everything living in it. The pool has had minimal use since opening this Spring.

We have been super shocking the pool all this week to try to get an unchanged chlorine reading through the night.

Our current levels as of 7:15 am on June 10, 2010 are: (we use FAS-DPD test kit)
FC = 16.5
CC = 0.5
pH = 7.6±
Alkal = 100
CYA = 50

(Just in case it does not appear in my signature - we have 10,000 gallon in ground vinyl lined pool with sand filter)

When we tested last night after dark the FC was 19.50 and CC = 1.0

This is the best FC hold we had over the past three nights. Previous readings were FC=14 night, and 10.5 morning. FC=15 night and 7.5 morning. CC has been mostly 0.5 all week with a couple tests up to 1 ppm during rain yesterday (the way our test works, one drop of the re-agent equates to CC=0.5 so that is the minimum we can read).

Everything we read indicates that to be sure all organisms in the water are killed, the FC should drop a maximum of 1.0 ppm overnight. The question is, with this much chlorine in the pool over the past 3-4 days (8 - 19.5 ppm depending on time of day and how much bleach we added) do you think we will ever get to the point where the FC drops a maximum of 1 ppm overnight???

Any suggestions on what else to do to get rid of the stain in the deep end would be appreciated also. One thing that I still would like to do to the stain but have yet to try is rubbing it with one of the rubbing stones that we purchased at the pool supply store last year.

Once we are done with this shock process we plan to keep the FC between 4 and 6 ppm and shock only when CC gets above 1.5 or so.

Thanks for any help you can offer.

Bama Rambler

Mod Squad
TFP Expert
LifeTime Supporter
Jun 22, 2009
SouthWest Alabama
First thing is that if you tested the water at 7:15 the sun was already up so it was consuming FC. So that's probably accounting for some of your loss. To get a good overnight test you need to test after the sun goes down and before the sun comes up. Yeah that means getting up real early! Keep shocking until you pass it.

The stains you see are most probably organic. You can try holding a trichlor puck directly on them for a little while and if they fade then keeping the FC at the high end of the recommended range after you're done shocking will help fade them over time.

Don't rub the vinyl liner with a rubbing stone as that'll remove the material and possibly ruin your liner. You can try a Magic Eraser but I'd try the trichlor puck first.


New member
Jul 14, 2009
Southeastern Pennsylvania
This is a follow up to post and reply of Thursday 06/10/2010

We added two pounds of powder shock around 7:45 pm last night. Did a water test at 9:15 pm after sun went down and got FC=27 and CC=0.5.

Got new water sample at 5:10 am this morning and did test around 7:15 am with FC=23 and CC=0.5.

We will keep at shock levels through today and try to get FC to hold overnight tonight into Saturday AM.

There are supposed to be folks coming over Saturday evening or Sunday morning to use the pool (family friend girls birthday party), so at some point I will need to end shock level and resume later if necessary should FC go down overnight again tonihgt.

Question is what do you think is max FC that pool can be safely used, and if it's a sunny day, how long do you think it will take to get from FC = 20 ppm to that max safe level.



Mod Squad
TFP Expert
LifeTime Supporter
Jul 10, 2009
Houston, Texas
You can swim up to shock level for your pool. For pool parties the recommendation is to raise the FC up a few ppm above normal daily level to handle the increased bather load. Test again after the party and bump up to the top of the daily FC level and run the overnight test to make sure you don't need to shock.


Platinum Supporter
LifeTime Supporter
Apr 14, 2010
Stephens City, VA
ssjl said:
We added two pounds of powder shock around 7:45 pm last night. Did a water test at 9:15 pm after sun went down and got FC=27 and CC=0.5.
You seem to be reading and have a grasp on the shocking process.. Please make sure you understand the effects of the powder shock and its active ingredients.

Dichlor powder shock will raise your CYA FAST, Cal Chlor will raise calcium.

I didn't read closely so if I missed this I'm sorry, on my way out the door and that one statement raised a flag. Have a great day and good luck!

Other Threads of Interest