How can I have algae when maintaining at 7ppm

simonoaks

LifeTime Supporter
May 28, 2015
318
Juno Beach , Florida
I do not have it running overnight.

Last night at 8.45 pm it was FC 11.5
at 6am this morning it was FC 10.

But I just realized I did not have pump running with SWG off. does that make a difference the overnight test?
I had same drop night before too.
Only 1.5 - plus of course there could easily be a .5 test discrepancy .
 

Richard320

TFP Expert
LifeTime Supporter
Jan 6, 2010
23,933
San Dimas, CA (LA County)
6pm last night FC 10.5
7.30 am this morning FC 9.5 (swg off, pump on )

Overnight test says it is all fine, BUT I am not sure water is QUITE as crystal clear as it could be , hard to tell. Does this mean anything ?
If you have any doubts that the algae is all killed or not, keep it up. If it is all gone, your chlorine losses will be minimal. And if it's not, you need to kill it.
 

simonoaks

LifeTime Supporter
May 28, 2015
318
Juno Beach , Florida
You lost 1ppm overnight. For my pool that would indicate issues. I lose less than 0.5 ppm overnight. So I say keep at it and see if your overnight losses drop.

Right , I know what you mean, but is was 1ppm from 6pm (2.5 hrs before sunset) and 7.30am (1 hour after sunrise) . That raises another point, is there a recommended amount of time to run the overnight for - e.g. is there a results difference from doing a test midnight then 4 am, over a test from 8pm to 6am ?
 

simonoaks

LifeTime Supporter
May 28, 2015
318
Juno Beach , Florida
7am today FC 9.5
8.40pm tonight with swg on normal 7 hr cycle FC 10
So pretty stable with swg at 62% . I am borderline as to whether I need to SLAM again, as it did pass after the SLAM a week ago. I also have a gut feeling that something is creeping back in, even though the daily and over night numbers do not support that.
 

simonoaks

LifeTime Supporter
May 28, 2015
318
Juno Beach , Florida
8.40 pm last night FC 10
6.30 am today FC 9
Still borderline
Added 1 gallon of 10%
7.30 am FC 15
4.30 pm FC 9.5 - but with SWG off AND filling 4 inches of water

As I suspected, that extra kick made the water crystal blue again - turns out it was SLIGHTLY off crystal , as thought, hard to tell when tiny difference. I can just make out the slight difference in clarity now. Hopefully that little kick did the job.

I cannot raise to SLAM as my 6 year old wants to swim this weekend and I do not want to expose her to FC over 15.
 

simonoaks

LifeTime Supporter
May 28, 2015
318
Juno Beach , Florida
Do you let her swim in indoor public/municipal pools at all?

Their active chlorine levels are typically higher than slam level.

Where does your 15 limit come from?

It is not an arbitrary limit. I use it as a 'safe ' level based on :

1. CYA test is known to be +/- 10-15
2. therefore SLAM is 18-28 (based on a CYA reading of 60-70 =/- 10-15
3. I feel that as I only lose FC 1 per night , I can kill anything left without going crazy
4. I actually do not know what the risks/ issues are with swimming over SLAM FC levels - what are they ?

Just to be sure, I added 1 gallon of 10% bleach at 6pm when we finished playing and leaving pump AND SWG on overnight.
8.45 pm - FC 18
Super sparkly pool.

I am also off to NYC next week , then back home to UK for 2 weeks , so will look into whether I should raise FC to like 40 or something the day before I leave, just to protect while I am away.
 

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cfherrman

TFP Guide
May 10, 2017
2,529
Hays, Kansas
A lot of backwashes will lower it, along with testing a the lower end of how filled is the the pool and testing at the high end of how filled it is. The cya test is also +/- 10 ppm if I recall.

I would test about a week later on a sunny day to confirm if results don't make sense.
 

simonoaks

LifeTime Supporter
May 28, 2015
318
Juno Beach , Florida
Ok, thx. I do not do backwashes.

I did test at low pool level , then the next test was after 4 inches of water. But my understanding is that CYA does not change based on water volume in pool. Certainly not 4 inches. I can rule out testing error as there is a HUGE difference between not sure whether it is 60-70 and when it is 30-40 . I probably see the dot even past 40.
 

CincyDools

Active member
May 18, 2017
44
Cincinnati, Ohio
Oh, I see, OK . thx

8.45 pm last night FC 18
9.45 am today - FC 19 (with SWG on overnight)
Pool looks crystal.
Will do another overnight to tonight

I am new to TFP as of last year and did not realize that safe swimming chlorine levels were dependant on CYA levels. Was not aware that it was safe to swim at slam levels. I guess I need to go back to Pool School! Are the safe levels discussed there?

This thread helps a lot for me because I was trying to let my FC levels go down to 5-7 PPM but like the OP was starting to see some algae once it got below 10 PPM. Sounds like maintaining at 10 PPM when my CYA is around 70 is safe for swimming.
 

cfherrman

TFP Guide
May 10, 2017
2,529
Hays, Kansas
Ok, thx. I do not do backwashes.

I did test at low pool level , then the next test was after 4 inches of water. But my understanding is that CYA does not change based on water volume in pool. Certainly not 4 inches. I can rule out testing error as there is a HUGE difference between not sure whether it is 60-70 and when it is 30-40 . I probably see the dot even past 40.

If you test cya at "full" and then at the bottom of the skimmer the low fill cya test would be higher, not by much (I'll throw out 5 ppm higher, not gonna do the math) and if you stack up all the other elements on why a cya test could be off it starts being plasuable.

You know your pool and you know what's reasonable, running your FC at 60 cya will be fine untill you test again even if your cya ends up at 30
 

Curtachio

Member
Jun 12, 2014
6
Annandale, NJ
I’m curious too how you made out. I have a form of what looks like a black algae and all my numbers are right on except CYA, I also have a difficult time with it. I did the overnight and FC went from 12 to 11. I drained ‘some’ water but I don’t think nearly enough (20% per pool calculator). I’m doing the overnight FC test again tonight and still trying to decide should I drain or SLAM?
 

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