Mysteriously disappearing chlorine

jellybean7

Active member
May 25, 2010
27
Over the last two weeks we started to lose chlorine in the pool. We have a floating chlorinator that had been keeping the chlorine level around 5, and I've been testing daily and weekly. Two weeks ago our pool turned milky overnight and the chlorine dropped to 0. I was able to clear up the pool in 24 hours by shocking it, but since then the chlorine keeps dropping to 0 every 3 days or so. I then add Clorox to bring the level back up, but it just goes back to 0 in a few days. The water looks crystal clear and we vacuum and brush it regularly. All other levels are where they need to be, including CYA, which has been at 40 for the last 6 weeks.

Where is the chlorine going? We go through 5-6 trichlor tablets a week.
 

JohnT

Admin
Mod Squad
TFP Expert
Apr 4, 2007
9,475
SW Indiana
I'm not sure 5 pucks a week is enough to chlorinate your pool.

If your pool is around 20,000 gallons, each puck will add about 2.75ppm to your pool. Since normal chlorine loss is 2-3ppm per day, you'd probably need 1 puck per day to keep up with chlorine consumption. Any biological load at all would pretty well wipe out your chlorine.
 

Bama Rambler

Mod Squad
TFP Expert
LifeTime Supporter
Jun 22, 2009
23,045
SouthWest Alabama
How are you testing?
Using 5 tablets a week is raising your CYA by 10ppm per week. I suspect that your CYA is much higher than you think it is.

You need to perform an OCLT (overnight chlorine loss test) to see what's going on in your pool.
 

jellybean7

Active member
May 25, 2010
27
I use the TF100 test kit and test chlorine and pH daily, everything else weekly. CYA has remained around 40 for weeks now. I use Clorox whenever I need to shock the pool, otherwise I use trichlor tablets I got at Costco. I know not all pool chemicals are created the same, but I was hoping that even generic chlorine tablets would do the job. I've done the overnight test and I never lose more than 1ppm.
 

anonapersona

TFP Expert
LifeTime Supporter
Nov 5, 2008
2,598
How hot has it been there? i suspect that the floater cannot dispense as fast as the sun is taking it.

When you test FC daily, do you then add Clorox to get it back up to the top of the range? Or are you hoping the chlorine tablets will do that overnight?
 

jellybean7

Active member
May 25, 2010
27
It's been pretty warm and sunny the last couple of weeks. Once the FC gets to around 1, I add enough Clorox to bring it up to around 5. If the tablets are not dissolving fast enough to replenish the chlorine, what is the best option? Should I add a second floater?
 

jellybean7

Active member
May 25, 2010
27
I stopped at the pool store today and the guy suggested that we might have an ammonia problem. Never really thought about it and i didn't have a water sample with me at the time to test it. what do i do with an ammonia problem? is there a way to test for it at home?
 

anonapersona

TFP Expert
LifeTime Supporter
Nov 5, 2008
2,598
Ammonia can be tested with a kit from the fish department of a pet store.

Who is using this pool? Dogs can take up a lot of chlorine. I have no idea how much chlorine was lost in my pool due to the 3 vultures at the hot tub but they left bits of tissue behind. Ducks tend to leave a lot of debris as well. Any chance a frog got sucked into the main drain? Or some other dead thing somewhere that needs cleaning out?
 

jellybean7

Active member
May 25, 2010
27
We have no pets, just 2 kids and small gatherings of people. We do have a lot of chipmunks and frogs in the yard and some tend to get in the pool and drown. I've not found anything dead in the pool in the last couple of weeks though. Haven't thought about something getting stuck in the drain though. I would hope that at some point that something would end up in the filter basket.

I shocked the pool again last night and tested overnight chlorine loss, which was less than 1ppm. CC is 0.5, which, as I understand, is related to possible ammonia content. I'll get an ammonia test kit today, just to be as thorough as possible.
 

frustratedpoolmom

TFP Expert
LifeTime Supporter
In The Industry
May 20, 2007
12,177
SWSuburban Chicago, IL
The thing with ammonia is the process to rid the pool of it is the same - you shock. So I'd skip the ammonia kit.

Usually in an ammonia situation the FC won't hold after gallons and gallons of bleach and it instantly reads as very high CC which you don't seem to be showing. This doesn't sound like an ammonia situation at all to me.

In any event, your FC is holding overnight, your CC is .5, (your water is clear, yes?) then you meet the criteria for being done shocking.

I just think the tablets can't keep up with the current demand that is going on in your pool. You may simply need to supplement with bleach, targeting a higher number at the start and just test more often and add bleach if you are getting too low.
 

jellybean7

Active member
May 25, 2010
27
Thank you,that all makes perfect sense. And yes, the water is clear. I knew there was a reason why I stopped going to the pool store! :rant: I'll keep monitoring the chlorine levels as I have and just add bleach as necessary.

As always, thank you everyone for your help!!